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.
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.