RIYADH (REUTERS) - Saudi Arabia executed a prominent Shi'ite Muslim cleric and dozens of Al-Qaeda members on Saturday, signalling it would not tolerate attacks, whether by Sunni militants or minority Shi'ites, and stirring sectarian anger across the region.

Hundreds of Shi'ite Muslims marched through Qatif district of 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 ruling Al Saud 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 in Saudi Arabia, 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.

Riyadh's main regional rival Iran and its Shi'ite allies immediately reacted with vigorous condemnation of the execution of Nimr, threatening Saudi Arabia and the Al Saud with severe repercussions, in protests ranging as far afield as India.

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 called for a severing of ties, calling into question fence-mending efforts by Riyadh that could help boost a regional alliance against Islamic State in Iraq and Syria (ISIS) militants.

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

ISIS called on Saturday on supporters to attack Saudi soldiers and police in revenge for the executions of militants, in a message on Telegram Channel, a prominent vehicle for 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 offences in Saudi Arabia since the 1980 killing of 63 militant rebels who seized Mecca's Grand Mosque in 1979.

ATTACKS

The 43 Sunni militants 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 Syariah 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".

Most extremist 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 Riyadh as an enemy.

Government-appointed clerics have for years denounced Al-Qaeda and ISIS as religious "deviants", while the government has cracked down on militants at home, squeezed their funding streams abroad and stopped them travelling to fight.

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

Mustafa Alani, a security analyst close to the Interior Ministry, commented: "There is a 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."

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.

That point appeared to be bolstered by the state-affiliated al Arabiya channel, which throughout Saturday showed graphics comparing Nimr and Faris al-Shuwail, an Al-Qaeda ideologue who was also executed, describing them both as "inciters".

ANGRY ACTIVISTS

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.

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.

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.

The executions are Saudi Arabia's first in 2016. At least 157 people were put to death last year, a big increase from the 90 people killed in 2014.