DUBAI/RIYADH, Jan 3 2016 - Iranian protesters stormed the Saudi Embassy in Tehran early on Sunday morning as Shi'ite Muslim Iran reacted with fury to Saudi Arabia's execution of a prominent Shi'ite cleric.

Demonstrators who had massed at the embassy gates to protest at Nimr al-Nimr's execution broke into the embassy and started fires before being cleared away by the police, Iran's ISNA news agency reported. Pictures were tweeted that showed parts of the interior on fire and smashed furniture inside one office.

Shortly afterwards, Iran's Foreign Ministry issued a statement calling for calm and urging protesters to respect the diplomatic premises, the Entekhab news website reported.

Iran's hardline Revolutionary Guards had promised "harsh revenge" against the Saudi Sunni royal dynasty for Saturday's execution of Nimr, considered a terrorist by Riyadh but hailed in Iran as a champion of the rights of Saudi Arabia's marginalized Shi'ite minority.

Nimr, the most vocal critic of the dynasty among the Shi'ite minority, had come to be seen as a leader of the sect's younger activists, who had tired of the failure of older, more measured leaders to achieve equality with Sunnis.

Although most of the 47 men killed in the kingdom's biggest mass execution for decades were Sunnis convicted of al Qaeda attacks in Saudi Arabia a decade ago, it was Nimr and three other Shi'ites, all accused of involvement in shooting police, who attracted most attention in the region and beyond.

The move appeared to end any hopes that the appearance of a common enemy in the form of the Islamic State militant force would produce some rapprochement between the region's leading Sunni and Shi'ite Muslim powers, who back opposing sides in wars currently raging in Syria and Yemen.

The website of Iran's Supreme Leader, Ayatollah Ali Khamenei, carried a picture of a Saudi executioner next to notorious Islamic State executioner 'Jihadi John', with the caption "Any differences?", and Iran's powerful Revolutionary Guards said "harsh revenge" would topple "this pro-terrorist, anti-Islamic regime."

Saudi Arabia summoned the Iranian ambassador, only to see its embassy stormed soon afterwards.

In Iraq, whose Shi'ite-led government is close to Iran, prominent religious and political figures demanded that ties with Riyadh be severed, calling into question Saudi attempts to forge a regional alliance against Islamic State, which controls swathes of Iraq and Syria.

Despite the regional focus on Nimr, the executions seemed mostly aimed at discouraging jihadism in Saudi Arabia, where dozens have died in the past year in attacks by Sunni militants.

The ruling Al Saud family has grown increasingly worried in recent years as Middle East turmoil, especially in Syria and Iraq, has boosted Sunni jihadists seeking to bring it down and given room to Iran to spread its influence. A nuclear deal with Iran backed by Saudi Arabia's biggest ally and protector, the United States, has done little to calm nerves in Riyadh.

But Saudi Arabia's Western allies, many of whom supply it with arms, are growing concerned about its new assertiveness in the region and at home.

The U.S. State Department said Nimr's execution "risks exacerbating sectarian tensions at a time when they urgently need to be reduced." The sentiment was echoed almost verbatim by EU foreign policy chief Federica Mogherini and an official at the German Foreign Ministry.

The State Department also urged the Saudi government to "respect and protect human rights, and to ensure fair and transparent judicial proceedings in all cases," as well as to permit peaceful expression of dissent and work with all community leaders to defuse tensions.

The simultaneous execution of 47 people - 45 Saudis, one Egyptian and a man from Chad - was the biggest mass execution for security offenses in Saudi Arabia since the 1980 killing of 63 jihadist rebels who seized Mecca's Grand Mosque in 1979.

The four Shi'ites had been convicted of involvement in shootings and petrol bomb attacks that killed several police during anti-government protests from 2011-13, in which more than 20 members of the minority sect were also shot dead by the authorities.

Family members of the executed Shi'ites have vigorously denied they were involved in attacks and said they were only peaceful protesters against sectarian discrimination.

Human rights groups have consistently attacked the kingdom's judicial process as unfair, pointing to accusations that confessions have been secured under torture and that defendants in court have been denied access to lawyers.

Riyadh denies torture and says its judiciary is independent.

Analysts have speculated that the execution of the four Shi'ites was partly to demonstrate to Saudi Arabia's majority Sunni Muslims that the government did not differentiate between political violence committed by members of the two sects.

The 43 Sunni jihadists executed on Saturday, including several prominent al Qaeda leaders and ideologues, were convicted for attacks on Western compounds, government buildings and diplomatic missions that killed hundreds from 2003-06.

"There is huge popular pressure on the government to punish those people," said Mustafa Alani, a security analyst close to the Saudi Interior Ministry. "It included all the leaders of al Qaeda, all the ones responsible for shedding blood. It sends a message."

Government-appointed clerics have for years denounced al Qaeda and Islamic State as religious "deviants," while the government has cracked down on jihadists at home, squeezed their funding streams abroad and stopped them traveling to fight.

Yet critics say the ruling family has not done enough to tackle the sectarian intolerance, hatred of infidels and praise for the principles of violent jihad propagated by Saudi clerics.

After the executions, Islamic State urged its supporters to attack Saudi soldiers and police in revenge, in a message on Telegram, an encrypted messaging service used by the group's backers, the SITE monitoring group reported.

Again, Saudi Arabia executed a prominent Shi'ite Muslim cleric and dozens of al Qaeda members on Saturday, signaling that it would not tolerate attacks, whether by Sunni jihadists or minority Shi'ites, and stirring sectarian anger across the region.

Hundreds of Shi'ite Muslims marched through Qatif district in Saudi Arabia's Eastern Province in protest at the execution of cleric Nimr al-Nimr, an eyewitness said. They chanted "Down with the Al Saud!", the name of the ruling Saudi royal family.

Nimr, the most vocal critic of the dynasty among the Shi'ite minority, had come to be seen as a leader of the sect's younger activists, who rejected the quiet approach of older community leaders for failing to achieve equality with Sunnis.

Most of the 47 killed in the kingdom's biggest mass execution for decades were Sunnis convicted of al Qaeda attacks in Saudi Arabia a decade ago. Four, including Nimr, were Shi'ites accused of involvement in shooting policemen.

The executions took place in 12 cities, four prisons using firing squads and the others beheading. In December, al Qaeda in the Arabian Peninsula threatened to retaliate against Saudi Arabia for any execution of its members.

The move further soured relations between Sunni-ruled Saudi Arabia and its Shi'ite regional rival, Iran, which had hailed Nimr as the champion of a marginalized Shi'ite minority.

The website of Iran's Supreme Leader, Ayatollah Ali Khamenei, carried a picture of a Saudi executioner next to notorious Islamic State executioner 'Jihadi John', with the caption "Any differences?", and the powerful Revolutionary Guards said "harsh revenge" would topple "this pro-terrorist, anti-Islamic regime." Saudi Arabia summoned the Iranian ambassador in response.

In Iraq, prominent religious and political figures demanded that ties be severed, calling into question fence-mending efforts by Riyadh that had been intended to boost a regional alliance against Islamic State militants.

However, the executions seemed mostly aimed at discouraging Saudis from jihadism after bombings and shootings by Sunni militants in Saudi Arabia over the past year killed dozens and Islamic State called on followers there to stage attacks.

After the executions, Islamic State urged its supporters to attack Saudi soldiers and police in revenge, in a message on Telegram, an encrypted messaging service used by the group's backers, the SITE monitoring group reported.

Saudi Arabia's ruling Al Saud family has grown increasingly worried in recent years as Middle East turmoil, especially in Syria and Iraq, has empowered Sunni militants seeking to bring it down and given room to Iran to spread its influence. A nuclear deal with Iran backed by Saudi Arabia's biggest ally, the United States, has done little to calm nerves in Riyadh.

But Saudi Arabia's Western allies, many of whom supply it with arms, are growing concerned about its new assertiveness.

EU foreign policy chief Federica Mogherini said Nimr's execution risked "dangerous consequences" by further inflaming sectarian tensions in the region, and a German Foreign Ministry official said it "strengthens our existing concerns about increasing tensions and deepening rifts in the region."

The simultaneous execution of 47 people - 45 Saudis, one Egyptian and a man from Chad - was the biggest mass execution for security offenses in Saudi Arabia since the 1980 killing of 63 jihadist rebels who seized Mecca's Grand Mosque in 1979.

The 43 Sunni jihadists executed on Saturday, including several prominent al Qaeda leaders and ideologues, were convicted for attacks on Western compounds, government buildings and diplomatic missions that killed hundreds from 2003-06.

The four Shi'ites were convicted of involvement in shootings and petrol bomb attacks that killed several police during anti-government protests from 2011-13 in which over 20 members of the minority sect were also shot dead by the authorities.

Under Saudi Arabia's reading of Islamic law, such attacks are interpreted as "banditry," carrying an automatic sentence of death followed by public display of bodies on gibbets.

Grand Mufti Abdulaziz Al al-Sheikh, the kingdom's top religious authority, praised the executions as "just." MOHAMMED AL-SHAIKH via Getty Images Bahraini women take part in a protest in the village of Jidhafs, west of Manama, against the execution of prominent Shiite Muslim cleric Nimr al-Nimr by Saudi authorities, on January 2, 2016.

Most jihadist groups follow a radical interpretation of the Salafi branch of Islam, the strict Sunni Muslim school that was developed in Saudi Arabia and is still followed by its clergy; but they have long regarded the U.S.-allied Saudi establishment as an enemy.

Government-appointed clerics have for years denounced al Qaeda and Islamic State as religious "deviants," while the government has cracked down on jihadists at home, squeezed their funding streams abroad and stopped them traveling to fight.

Mustafa Alani, a security analyst close to the Interior Ministry, commented: "There is huge popular pressure on the government to punish those people. It included all the leaders of al Qaeda, all the ones responsible for shedding blood. It sends a message."

However, critics of the ruling family say it has not done enough to tackle the sectarian intolerance, hatred of infidels and praise for the principles of violent jihad propagated by Saudi clerics, which they see as contributing to militancy.

Analysts have speculated that the execution of the four Shi'ites was partly to demonstrate to Saudi Arabia's majority Sunni Muslims that the government did not differentiate between political violence committed by members of the two sects.

Human rights groups have consistently attacked the kingdom's judicial process as unfair, pointing to accusations that confessions have been secured under torture and that defendants in court have been denied access to lawyers.

Family members of the executed Shi'ites have vigorously denied they were involved in attacks and said they were only peaceful protesters against sectarian discrimination in the Sunni-ruled kingdom.

Riyadh denies torture and says its judiciary is independent.

The cleric's brother, Mohammed al-Nimr, said he hoped any response in Qatif would be peaceful, but activists said new protests were possible.

"My mobile is getting non-stop messages from friends, all shocked and angry ... The fear is for the children among those detained," an activist in Qatif told Reuters.

The Interior Ministry statement began with Koranic verses justifying the use of execution, and state television showed footage of the aftermath of al Qaeda attacks in the last decade.

At least 157 people were put to death in Saudi Arabia last year, a big increase from the 90 executed in 2014.

Their sons shall wage war and assemble a multitude of great forces, which shall come on and overflow and pass through, and again shall carry the war as far as his fortress.

Then the king of the south, moved with anger, shall come out and fight with the king of the north; and he shall raise a great multitude, but it shall be given into his hand.

And when the multitude is taken, his heart shall be exalted, and he shall cast down tens of thousands, but he shall not prevail.

For the king of the north shall again raise a multitude, greater than the former; and after some years he shall come on with a great army and abundant supplies.

In those times many shall rise against the king of the south; and the men of violence among your own people shall lift themselves up in order to fulfil the vision; but they shall fail.

Then the king of the north shall come and throw up siegeworks, and take a well-fortified city. And the forces of the south shall not stand, or even his picked troops, for there shall be no strength to stand.

But he who comes against him shall do according to his own will, and none shall stand before him; and he shall stand in the glorious land, and all of it shall be in his power.

He shall set his face to come with the strength of his whole kingdom, and he shall bring terms of peace and perform them. He shall give him the daughter of women to destroy the kingdom; but it shall not stand or be to his advantage.

Afterward he shall turn his face to the coastlands, and shall take many of them; but a commander shall put an end to his insolence; indeed he shall turn his insolence back upon him.

Then he shall turn his face back toward the fortresses of his own land; but he shall stumble and fall, and shall not be found.

Then shall arise in his place one who shall send an exactor of tribute through the glory of the kingdom; but within a few days he shall be broken, neither in anger nor in battle.

In his place shall arise a contemptible person to whom royal majesty has not been given; he shall come in without warning and obtain the kingdom by flatteries.

Armies shall be utterly swept away before him and broken, and the prince of the covenant also.

And from the time that an alliance is made with him he shall act deceitfully; and he shall become strong with a small people.

Without warning he shall come into the richest parts of the province; and he shall do what neither his fathers nor his fathers' fathers have done, scattering among them plunder, spoil, and goods. He shall devise plans against strongholds, but only for a time.

And he shall stir up his power and his courage against the king of the south with a great army; and the king of the south shall wage war with an exceedingly great and mighty army; but he shall not stand, for plots shall be devised against him.

Even those who eat his rich food shall be his undoing; his army shall be swept away, and many shall fall down slain.

And as for the two kings, their minds shall be bent on mischief; they shall speak lies at the same table, but to no avail; for the end is yet to be at the time appointed.

And he shall return to his land with great substance, but his heart shall be set against the holy covenant. And he shall work his will, and return to his own land.

At the time appointed he shall return and come into the south; but it shall not be this time as it was before.

For ships of Kittim shall come against him, and he shall be afraid and withdraw, and shall turn back and be enraged and take action against the holy covenant. He shall turn back and give heed to those who forsake the holy covenant.

Forces from him shall appear and profane the temple and fortress, and shall take away the continual burnt offering. And they shall set up the abomination that makes desolate.

He shall seduce with flattery those who violate the covenant; but the people who know their God shall stand firm and take action.

And those among the people who are wise shall make many understand, though they shall fall by sword and flame, by captivity and plunder, for some days.

When they fall, they shall receive a little help. And many shall join themselves to them with flattery;

and some of those who are wise shall fall, to refine and to cleanse them and to make them white, until the time of the end, for it is yet for the time appointed.

And the king shall do according to his will; he shall exalt himself and magnify himself above every god, and shall speak astonishing things against the God of gods. He shall prosper till the indignation is accomplished; for what is determined shall be done.

He shall give no heed to the gods of his fathers, or to the one beloved by women; he shall not give heed to any other god, for he shall magnify himself above all.

He shall honor the god of fortresses instead of these; a god whom his fathers did not know he shall honor with gold and silver, with precious stones and costly gifts.

He shall deal with the strongest fortresses by the help of a foreign god; those who acknowledge him he shall magnify with honor. He shall make them rulers over many and shall divide the land for a price.

At the time of the end the king of the south shall attack him; but the king of the north shall rush upon him like a whirlwind, with chariots and horsemen, and with many ships; and he shall come into countries and shall overflow and pass through.

He shall come into the glorious land. And tens of thousands shall fall, but these shall be delivered out of his hand: Edom and Moab and the main part of the Ammonites.

He shall stretch out his hand against the countries, and the land of Egypt shall not escape.

He shall become ruler of the treasures of gold and of silver, and all the precious things of Egypt; and the Libyans and the Ethiopians shall follow in his train.

But news from the east and the north shall alarm him, and he shall go forth with great fury to exterminate and utterly destroy many.

And he shall pitch his palatial tents between the sea and the glorious holy mountain; yet he shall come to his end, with none to help him.