Saudi Arabia broke off diplomatic ties with Iran after protesters ransacked its embassy in Tehran to protest at the execution of a Shi'ite cleric whose killing has sparked fury.
Saudi Foreign Minister Adel al-Jubeir made the announcement in Riyadh, saying Iranian diplomats had 48 hours to leave the kingdom, but Tehran fired back that Saudi Arabia's decision would not distract from its "big mistake" of executing Sheikh Nimr al-Nimr.
Relations between Sunni-ruled Saudi Arabia and Shi'ite-dominated Iran have been strained for decades, with Riyadh frequently accusing Tehran of interfering in Arab affairs.
The oil-rich foes have also been divided over the nearly five-year war in Syria, where Iran is backing the regime, and the conflict in Yemen where a Saudi-led coalition is battling Shi'ite rebels.
The spike in tensions, which comes after Iran last year secured a historic nuclear deal with world powers led by the United States, saw oil prices rise Monday in Asian trading.
On Sunday, Iran's supreme leader said Riyadh would face "quick consequences" for executing Nimr, as Washington urged regional leaders to soothe escalating sectarian tensions between Sunni and Shi'ite Muslims.
Saudi Arabia "is breaking off diplomatic ties with Iran and requests that all members of the Iranian diplomatic mission leave… within 48 hours," Jubeir said.
"Iran's history is full of negative interference and hostility in Arab issues, and it is always accompanied by destruction," he said, accusing Tehran of seeking to "destabilize" the region.
On Saturday, a mob attacked the Saudi embassy in Tehran and a consulate in Mashhad, Iran's second-biggest city, amid protests at Nimr's execution.
Jubeir said Saudi authorities had asked their Iranian counterparts to ensure security at the embassy but they did not cooperate and failed to protect it.
Nimr, 56, was a force behind 2011 anti-government protests in oil-rich eastern Saudi Arabia, where Shi'ites have long complained of marginalization.
He was put to death along with 46 other people, including Shi'ite activists and convicted Sunni militants who the Saudi interior ministry says were involved in Al-Qaeda attacks that killed dozens in 2003 and 2004.
Some were beheaded and others were executed by firing squad.
'Big mistake'
Iran has said it arrested 44 people over the embassy attacks, and President Hassan Rouhani said the demonstrators were "radicals" and the assaults "totally unjustifiable".
Iran's supreme leader Ayatollah Ali Khamenei, meanwhile, condemned Nimr's execution, saying "God will not forgive" Saudi Arabia for putting him to death.
"The unjustly spilt blood of this martyr will have quick consequences," he said, adding: "It will haunt the politicians of this regime."
Deputy foreign minister Hossein Amir Abdollahian later said, according to IRNA: "By deciding to sever (diplomatic) relations, Saudi Arabia cannot make (the world) forget its big mistake of executing a cleric."
Khamenei was joined in his condemnation of Nimr's execution by Iraq's top Shi'ite authority, Grand Ayatollah Ali al-Sistani, who called the death sentence "an unjust act of aggression".
Their comments, echoed by other regional religious and political leaders, came as protests in Iran on Sunday spread to Bahrain, Pakistan, Indian Kashmir and Lebanon.
Saudi Arabia branded Nimr an "instigator of sedition" and arrested him in 2012, after a video on YouTube showed him making a speech celebrating the death of the then-interior minister.
Three years earlier he called for the oil-rich Eastern Province's Shi'ite-populated Qatif and Al-Ihsaa governorates to be separated from Saudi Arabia and united with Bahrain.
'Gates of hell'
In Eastern Province on Sunday, state media said Saudi police were shot at in a "terrorist action" in Nimr's home village. A civilian was killed and a child wounded, but there were no details on whether there were any police casualties.
Demonstrations outside the Saudi embassy and at Palestine Square in Tehran attracted around 1,500 people Sunday, with chants of "Death to the House of Saud".
On Baghdad's Palestine Street, Iraqi cleric Ahmed al-Shahmani said: "The House of Saud has opened the gates of hell on its own regime."
In Bahrain, where authorities defended Saudi Arabia along with other Gulf allies of Riyadh, police used buckshot and tear gas against Shi'ite protesters who threw petrol bombs. Arrests were reported.
Nimr's execution was widely condemned elsewhere by major Western powers, and the United States on Sunday called on Middle East countries to take "affirmative steps" to calm tensions.
Executions have soared in Saudi Arabia since King Salman ascended the throne a year ago with 153 people put to death in 2015, nearly twice as many as in 2014, for crimes ranging from murder to drug trafficking, armed robbery, rape and apostasy.
Human Rights Watch said the mass execution was the largest since 1980, and called it a "shameful start to 2016", while Amnesty International said Riyadh was using Nimr's execution "to settle political scores".
