Police managed to restore order after protesters stormed a Saudi embassy in Tehran, throwing papers from the roof and petrol bombs at the building early on Sunday.
"The fire has destroyed the interior of the embassy," an eyewitness told the AFP news agency. "The police are everywhere and have dispersed the demonstrators, some of whom have been arrested."
Firefighters were at the scene and dousing the fires, according to reports.
UN chief outraged
United Nations Secretary General Ban Ki-Moon has said on Saturday that he was "deeply dismayed" by the execution of 47 people in Saudi Arabia.
"The Secretary-General also calls for calm and restraint in reaction to the execution of Sheikh Nimr and urges all regional leaders to work to avoid the exacerbation of sectarian tensions," Ban's spokesman said.
He "deplores the violence by demonstrators against the Saudi Embassy in Tehran," Ban's spokesman added.
Foreign Ministry spokesman Hossein Jaber Ansari also urged calm and said there should be no more rallies around Saudi diplomatic premises, the state news agency IRNA reported.
Angry crowds also set fire to a Saudi consulate in Mashhad, Iran's second biggest city, according to media reports.
Death of cleric rises tensions
The unrest started after Riyadh announced the execution of prominent Shiite cleric Nimr al-Nimr on terrorism charges. Nimr was executed on Saturday alongside 46 other Sunni and Shiite men.
The move sparked international outrage and strong protests from Iraq and Iran, with the Shiite-majority Iran saying that the charges were politically and religiously motivated.
Tehran also accused the Sunni kingdom of supporting "terrorist movements and extremists" while using "oppression and execution" to silence its domestic critics. Iran's foreign ministry warned that Riyadh would "pay a high price for following these policies."
In response, Saudi Arabia summoned Iran's envoy and handed him "a stern protest letter over the aggressive Iranian statements."
dj,smm/bk (Reuters, AFP, dpa)