Protesters stormed the Saudi embassy in Tehran, burning images of the Saudi king. Photo: EPA
Saudi Arabia will face "divine revenge" for its execution of a prominent Shia cleric, Iran's supreme leader Ayatollah Ali Khamenei has warned.
Ayatollah Khamenei described Sheikh Nimr al-Nimr as a "martyr" who acted peacefully. Protesters stormed the Saudi embassy in Tehran late on Saturday, setting fire to the building before being driven back by police.
Sheikh Nimr was one of 47 people executed for terrorism offences. But Ayatollah Khamenei said the cleric had been executed for his opposition to Saudi Arabia's Sunni rulers.
"This oppressed scholar had neither invited people to armed movement, nor was involved in covert plots," the ayatollah tweeted.
"The only act of #SheikhNimr was outspoken criticism," he added, saying the "unfairly-spilled blood of oppressed martyr #SheikhNimr will affect rapidly & Divine revenge will seize Saudi politicians".
Sheikh Nimr had been a figurehead in the anti-government protests that erupted in the wake of the Arab Spring up to his arrest in 2012.
Iran - Saudi Arabia's main regional rival - has led condemnation among Shia communities over the execution.
The foreign ministry in Tehran said the Sunni kingdom would pay a high price for its action, and it summoned the Saudi charge d'affaires in Tehran in protest.
Some of the protesters at the Saudi embassy in Tehran hurled petrol bombs and rocks. Forty people have been arrested, officials said.
There have also been demonstrations in Saudi Arabia's Eastern Province, where Shia Muslims complain of marginalisation, as well as in Iraq, Bahrain and several other countries.
For its part, Saudi Arabia complained to the Iranian envoy in Riyadh about what it called "blatant interference" in its internal affairs.
The US and UN have both called for restraint.
In a statement after the executions, US state spokesman John Kirby appealed to Saudi Arabia's government to respect and protect human rights, and to ensure fair and transparent judicial proceedings.
Mr Kirby also urged the Saudi government to permit peaceful expression of dissent and, along with other leaders in the region, to redouble efforts to reduce regional tensions.