Saudi Arabia has executed the prominent Shia cleric Sheikh Nimr al-Nimr, the interior ministry said.
He was among 47 people put to death after being convicted of terrorism offences, it said in a statement.
Sheikh Nimr was a vocal supporter of the mass anti-government protests that erupted in Eastern Province in 2011, where a Shia majority have long complained of marginalisation.
Shia-led Iran said Saudi Arabia would pay a “high price” for the execution.
A foreign ministry spokesman said Riyadh “supports terrorists… while executing and suppressing critics inside the country”.
Iranian state TV reported that the Saudi charge d’affaires in Iran had been summoned to the foreign ministry.
Saudi Embassy in Tehran getting the kind of attention the U.S. Embassy used to. pic.twitter.com/YBA3qs1Ewg
— ian bremmer (@ianbremmer) January 2, 2016
Iran is the main regional rival of Sunni-ruled Saudi Arabia.
At least one protest march was held in Qatif, in Eastern Province, where security has been raised.
Protesters shouted the slogans “The people want the fall of the regime”, and “Down with the al-Saud family”, reminiscent of the 2011 protests.
The BBC understands that among those executed was a man convicted of shooting dead a freelance cameraman on an assignment for the BBC, Simon Cumbers, in 2004.
Adel al-Dubayti was sentenced in November 2014 for his role in multiple al-Qaeda attacks including the one in the capital Riyadh in which Cumbers was killed and which also left reporter Frank Gardner critically injured.
The executions were carried out simultaneously in 12 locations across Saudi Arabia.
Those also put to death include Sunnis convicted of involvement in al-Qaeda-linked terror attacks in 2003. Of the 47 executed, one was a Chadian national while another was Egyptian. The rest are Saudis.
Saudi Arabia’s top cleric, Grand Mufti Sheikh Abdulaziz Al al-Sheikh, defended the executions, calling them a “mercy to the prisoners” as it would prevent them committing more crimes, Associated Press reported.
According to the ISNA, the Iranian Students News Agency, some demonstrators reached the roof of the embassy before being cleared. There were also reports that the Saudi consulate in Mashhad, Iran’s second largest city, had been set alight.
Protesters break into Saudi embassy building in Tehran pic.twitter.com/7wtBGpZuco
— Sobhan Hassanvand (@Hassanvand) January 2, 2016