Iranian protesters have stormed the Saudi Embassy in Tehran and Shi'ite Muslim Iran's top leader predicted "divine vengeance" for Saudi Arabia's execution of a prominent Shi'ite cleric.
Demonstrators, massed at the embassy gates to protest against cleric Nimr al-Nimr's execution, broke into the building, smashed furniture and started fires before being ejected by police.
Iran's foreign ministry called for calm and urged protesters to respect the diplomatic premises.
Tehran's police chief said that an unspecified number of "unruly elements" had been arrested for attacking the embassy with petrol bombs and rocks.
A prosecutor said 40 people were arrested, state media said.
Iranian Supreme Leader Ayatollah Ali Khamenei, criticising Saudi Arabia for the second straight day over Nimr's execution, said politicians in the Sunni kingdom would face divine retribution for his death.
"The unjustly spilled blood of this oppressed martyr will no doubt soon show its effect and divine vengeance will befall Saudi politicians," state TV reported Khamenei as saying.
Iran's Revolutionary Guards had promised "harsh revenge" against the Saudi Sunni royal dynasty for Saturday's execution of Nimr, considered a terrorist by Riyadh but hailed in Iran as a champion of the rights of Saudi Arabia's marginalised Shi'ite minority.
Nimr, the most vocal critic of the dynasty among the Shi'ite minority, had come to be seen as a leader of the sect's younger activists, who had tired of the failure of older, more measured leaders to achieve equality with Sunnis.
Although most of the 47 men killed in the kingdom's biggest mass execution for decades were Sunnis convicted of al Qaeda attacks in Saudi Arabia a decade ago, it was Nimr and three other Shi'ites, all accused of involvement in shooting police, who attracted most attention in the region and beyond.
The move appeared to end any hopes that the appearance of a common enemy in the form of the Islamic State militant force would produce some rapprochement between the region's leading Sunni and Shi'ite Muslim powers, allied to opposing sides in wars currently raging in Syria and Yemen.
Khamenei's website carried a picture of a Saudi executioner next to notorious IS executioner Jihadi John, with the caption "Any differences?". The Revolutionary Guards said "harsh revenge" would topple "this pro-terrorist, anti-Islamic regime".
In Iraq, whose Shi'ite-led government is close to Iran, prominent 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.
The US State Department said Nimr's execution "risks exacerbating sectarian tensions at a time when they urgently need to be reduced".
The sentiment was echoed almost verbatim by EU foreign policy chief Federica Mogherini and an official at the German Foreign Ministry.
The State Department also urged the Saudi government to "respect and protect human rights, and to ensure fair and transparent judicial proceedings in all cases", as well as to permit peaceful expression of dissent and work with all community leaders to defuse tensions.
