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-lead 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”.
Iran is the main regional rival of Sunni-ruled Saudi Arabia.
There are reports of protests in Qatif, in Eastern Province, where security has been raised.
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.
The international rights group Reprieve called the executions “appalling”, saying at least four of those killed, including Sheikh Nimr, were put to death for offences related to political protest.
Protests broke out in early 2011 in the oil-rich Eastern Province in the wake of the Arab Spring.
Sheikh Nimr’s arrest in the following year, during which he was shot, triggered days of protests in which three people were killed.
His death sentence was confirmed in October 2014, with his family saying he had been found guilty among other charges of seeking “foreign meddling” in the kingdom.
Sheikh Nimr’s supporters say he advocated only peaceful demonstrations and eschewed all violent opposition to the government.
The cleric’s nephew, Ali al-Nimr, who was 17 when he was arrested following the demonstrations and also faces execution, was not listed as one of those killed.
His brother, Mohammed al-Nimr, said he hoped any reaction to the execution would be peaceful.
But an MP in Iraq’s governing Shia coalition said the death aimed at “provoking sectarian fighting”, while Lebanon’s Shia council called it a “grave mistake”, Reuters reported.
Police in Bahrain, which has seen tensions between the majority Shia population and its Sunni rulers, fired tear gas on protesters angry at the execution.
Saudi authorities deny discriminating against Shia and blame Iran for stirring up discontent.
Saudi Arabia carried out more than 150 executions last year, the highest figure recorded by human rights groups for 20 years.