Dubai/Riyadh: Iranian protesters stormed the Saudi embassy in Tehran early on Sunday after Saudi Arabia executed a prominent cleric.
Strong rhetoric from Tehran was matched by others across the region, with Sayyed Hassan Nasrallah, the head of Lebanon's Hezbollah movement, describing the execution as "a message of blood" and Moqtada Al Sadr, an Iraqi cleric, called for angry protests.
Iranian demonstrators protesting against the execution of cleric Sheikh Nimr Al Nimr broke into the embassy building, smashed furniture and started fires before being ejected by police.
Iran's President Hassan Rouhani condemned the execution as "inhuman", but also urged the prosecution of "extremist individuals" for attacking the embassy and the Saudi consulate in the northeastern city of Mashhad, state media reported.
Tehran's police chief said an unspecified number of "unruly elements" were arrested for attacking the embassy with petrol bombs and rocks. A prosecutor said 40 people were held.
The Saudi and Iranian foreign ministers told Austria they have no interest in a further of heightening tensions between them, a spokesman for Austria's Foreign Minister Sebastian Kurz said.
Nimr execution sparked angry protests in the Qatif region in eastern Saudi Arabia.
Relatives of Nimr, reached by telephone, said authorities have informed them that the body had been buried "in a cemetery and would not be handed over to the family.
Although most of the 47 men killed in the kingdom's biggest mass execution for decades were convicted of Al Qaeda attacks in Saudi Arabia a decade ago, it was Nimr and three others, all accused of involvement in shooting police, who attracted most attention in the region and beyond.
Saudi Arabia on Saturday summoned the Iranian ambassador to protest what it described as hostile remarks emerging from Tehran. On Sunday, the United Arab Emirates, Kuwait and Bahrain also summoned Tehran's envoys to their countries to lodge complaints.
In Iraq, religious and political figures demanded that ties with Riyadh be severed, calling into question Saudi attempts to forge a regional alliance against IS, which controls swaths of Iraq and Syria.
Iraq's top cleric Grand Ayatollah Ali Al Sistani described the executions as an "unjust aggression".
Despite the focus on Nimr, the executions seemed mostly aimed at discouraging militancy in Saudi Arabia, where dozens have died in the past year in attacks by militants.
But Western countries are growing concerned about Saudi's new assertiveness.
The US State Department said Nimr's execution "risks exacerbating sectarian tensions at a time when they urgently need to be reduced", a sentiment echoed by EU foreign policy chief Federica Mogherini. The State Department also urged Saudi Arabia to respect and protect human rights.
France said on Sunday it deeply deplored the mass execution and said it reiterated its opposition to the death penalty in all circumstances.
In Istanbul, hundreds of protesters, some carrying pictures of Nimr and chanting slogans gathered outside Saudi consulate on Sunday as riot police stood guard.
The four had been convicted of involvement in shootings and petrol bomb attacks that killed several police during protests from 2011-13. More than 20 were shot dead by the authorities in those protests.