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Iran's Supreme Leader Ayatollah Ali Khamenei warned of repercussions after Saudi Arabia executed a prominent Shiite cleric critical of the kingdom's Sunni rulers.
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Iranian protesters set fire to the Saudi Embassy in Tehran, as Shi'ite Muslim Iran reacts with fury to Saudi Arabia's execution of a prominent Shi'ite cleric.
"The divine hand of revenge will take the Saudi politicians by the throat," Khamenei, Iran's highest authority, said on Sunday.
Cleric Nimr al-Nimr "was neither encouraging people to armed protests, nor plotting secretly, all he did was to openly criticise".
Iranian protesters armed with rocks and firebombs attacked the Saudi embassy in Tehran on Saturday and set parts of the building on fire after the execution of al-Nimr, an outspoken critic of the kingdom's treatment of its Shiite minority.
A small group stormed the premises and several people were arrested, Tehran police chief Hossein Sajedinia told the state-run Islamic Students' News Agency.
Al-Nimr's execution and the ensuing protests further strained ties between Saudi Arabia and Shiite-ruled Iran.
The two regional powers are on opposite sides of Middle East conflicts from Syria to Yemen.
Saudi Arabia is also concerned about Iran's growing influence after last year's nuclear accord with world powers.
The cleric was one of 47 executed across Saudi Arabia on Saturday for terrorism-related offences.
The men were convicted of crimes including bombings that targeted the traffic department and interior ministry in Riyadh, plots to attack military airports, and other strikes on security forces.
"This sends a message of resolve and firmness in Saudi Arabia's policy of confrontation with its two enemies, mainly al-Qaeda and Iran," Ibrahim Fraihat, senior foreign policy fellow at the Brookings Doha Centrte, said in an interview.
"It will also increase tensions inside Saudi Arabia because it gives the Shiite community new grievances and symbols to rally around within the country."
Some of the attacks happened between 2003 and 2006. The executed were described as promoters of a "deviant" version of Islam, a phrase used by Saudi Arabia for al-Qaeda, Islamic State and other militant groups. Most of those executed were Sunnis.
Mansour al-Turki, a spokesman for Saudi Arabia's Interior Ministry, said the kingdom "doesn't pay attention to external threats or comments made by officials in other countries about judicial measures and procedures in Saudi Arabia".
Al-Nimr was arrested in 2012 and sentenced to death in 2014. In 2009, he threatened to lead Saudi Arabia's Shiite Muslims to secession, provoking a government crackdown in the minority's eastern heartland, the centre of the kingdom's oil reserves. Shiites make up 10 to 15 per cent of Saudi Arabia's population, according to the CIA World Factbook.
His family, in a statement posted on Twitter, called upon Shiites "to have self-control" and to continue "to demand their rights peacefully".
While Saudi Arabia largely escaped the unrest that spread across the Arab world in 2011, the country's minority Shiites, who say they suffer discrimination, have occasionally protested and fought with security forces.
Islamic State militants have exploited this fault line, striking Shiite mosques last year.
"We are particularly concerned that the execution of prominent Shia cleric and political activist Nimr al-Nimr risks exacerbating sectarian tensions at a time when they urgently need to be reduced," US State Department spokesman John Kirby said in a statement.
"We have previously expressed our concerns about the legal process in Saudi Arabia and have frequently raised these concerns at high levels of the Saudi government."
Iran has summoned Ahmed Maloud, Saudi Arabia's charge d'affaires in Tehran, to protest at the execution, according to its state-run Fars news agency.
Hezbollah, the Lebanese militant group backed by Iran, said that the US was directly responsible for al-Nimr's execution because it supports the Saudi Arabian government.
The punishments were carried out in 10 provinces, Saudi Arabia's interior ministry said in a statement published on Saturday by the official Saudi Press Agency.
While most of the convicted men were Saudi citizens, an Egyptian and a Chadian national were among those executed.
Some of the men were shot by a firing squad and some were beheaded, a ministry spokesman said on Al-Arabiya TV.
Iraq's top Shiite cleric Grand Ayatollah Ali al-Sistani on Sunday condemned the execution of Al-Nimr, calling it an "unjust agression".
"We have received with much sorrow and regret the news of the martyrdom of a number of our brother believers in the region whose pure blood was shed in an unjust aggression," the cleric said in a letter addressed to the population of the eastern Saudi region of Qatif where al-Nimr used to preach.
The opinion of Sistani, based in the Shiite holy city of Najaf south of Baghdad, carries weight with millions of Shiites in Iraq and elsewhere.