The execution of a Shiite cleric in Sunni-ruled Saudi Arabia has laid bare divisions in the Middle East.
Protesters set fire to the kingdom's embassy in Tehran and demonstrators took to the streets from Bahrain to Pakistan after the mass execution of Sheikh Nimr al-Nimr and 46 others.
It was the largest execution carried out by Saudi Arabia in three and a half decades and illustrates the kingdom's new aggressiveness under King Salman.
Under his reign, Saudi Arabia has led a coalition fighting Shiite rebels in Yemen and staunchly opposed regional Shiite power Iran, even as Tehran struck a nuclear deal with world powers.
Iran's top leader warned Saudi Arabia on Sunday of "divine revenge" over Sheikh al-Nimr's death, while Riyadh accused Tehran of supporting terrorism in a war of words that threatened to escalate even as the US and the European Union sought to calm the region.
Sheikh al-Nimr was a central figure in Arab Spring-inspired protests by Saudi Arabia's Shiite minority until his arrest in 2012. He was convicted of terrorism charges but denied advocating violence.
On Saturday, Saudi Arabia put Sheikh al-Nimr and three other Shiite dissidents to death, along with a number of al Qaida militants. Sheikh al-Nimr's execution drew protests from Shiites across the world, who backed his call for reform and wider political freedom for their sect.
While the split between Sunnis and Shiites dates back to the early days of Islam and disagreements over the successor to Prophet Mohammed, those divisions have only grown as they intertwine with regional politics today, with both Iran and Saudi Arabia vying to be the Middle East's top power.
Iran accuses Saudi Arabia of supporting terrorism in part because it backs Syrian rebel groups fighting to oust its embattled ally, President Bashar Assad. Riyadh points to Iran's backing of the Lebanese Hezbollah and other Shiite militant groups in the region as a sign of its support for terrorism. Iran also has backed Shiite rebels in Yemen known as Houthis.
Iran's supreme leader, Ayatollah Ali Khamenei, condemned Sheikh al-Nimr's execution, saying on Sunday the cleric "neither invited people to take up arms nor hatched covert plots. The only thing he did was public criticism."
Iran's powerful Revolutionary Guard said Saudi Arabia's "medieval act of savagery" would lead to the "downfall" of the country's monarchy.
Saudi Arabia's Foreign Ministry said that by condemning the execution, Iran had "revealed its true face represented in support for terrorism".
In Tehran, a protest outside the Saudi Embassy early on Sunday quickly grew violent.
Protesters threw stones and petrol bombs at the embassy, setting part of the building ablaze, according to General Hossein Sajedinia, the country's top police official, the semi-official Tasnim news agency said.
Forty people were arrested and investigators were pursuing other suspects, Tehran prosecutor Abbas Jafari Dowlatabadi said, according to the semi-official ISNA news agency.
Iranian president Hassan Rouhani condemned Saudi Arabia's execution of Sheikh al-Nimr, but also branded those who attacked the Saudi Embassy as "extremists".
"It is unjustifiable," he said in a statement.
Hundreds of protesters later demonstrated in front of the embassy and in a central Tehran square. Street signs near the embassy were replaced with ones bearing the slain sheikh's name.
Protesters also took to the streets across the region.
In Bahrain, police used water cannons and fired birdshot at demonstrators on Sitra Island, south of the capital, Manama, wounding some. In al-Daih, west of the capital, Shiite protesters chanted against Saudi Arabia's ruling Al Saud family, as well as against Bahrain's ruling Al Khalifa family.
In Beirut, Hezbollah leader Hassan Nasrallah called Sheikh al-Nimr "the martyr, the holy warrior", while demonstrators marched in Turkey, India and Pakistan.
Western powers sought to calm the tensions. In Washington, State Department spokeswoman Elizabeth Trudeau said the US condemned the embassy attack and called on all sides "to avoid any actions that would further heighten tensions in the region".
European Union foreign policy chief Federica Mogherini spoke to Iranian foreign minister Mohammad Javad Zarif by phone and urged Tehran to "defuse the tensions and protect the Saudi diplomats", according to a statement.
The cleric's execution has threatened to complicate Saudi Arabia's relationship with the Shiite-led government in Iraq, where the Saudi Embassy is preparing to formally reopen for the first time in nearly 25 years. On Saturday there were calls for the embassy to be shut down again.
Iran and Saudi Arabia summoned each other's envoys for consultations, and Saudi allies Egypt and the United Arab Emirates summoned Iranian officials in their capitals over the Tehran embassy assault.
Meanwhile, Sheikh al-Nimr's family prepared for three days of mourning at a mosque in al Awamiya in the kingdom's al Qatif region in predominantly Shiite eastern Saudi Arabia.
The sheikh's brother, Mohammed al-Nimr, said that Saudi officials informed his family that the cleric had been buried in an undisclosed cemetery, a development that could lead to further protests.