Saudi Arabia's execution Saturday of a prominent Shi'ite cleric and 46 other men prompted outrage in parts of the Middle East, with Iran warning Riyadh would pay a "high price."
But several Saudi allies offered their support after Riyadh implemented death sentences against cleric Nimr al-Nimr, Shiite activists and Sunnis accused of involvement in Al-Qaeda attacks.
Iran's foreign ministry spokesman Hossein Jaber Ansari strongly condemned Riyadh for executing Nimr despite repeated Iranian requests for clemency.
"The Saudi government supports terrorist movements and extremists, but confronts domestic critics with oppression and execution… The Saudi government will pay a high price for following these policies," he said, quoted by the official IRNA news agency.
"The execution of a figure like Sheikh al-Nimr, who had no means to follow his political and religious goals but through speaking out, merely shows the extent of irresponsibility and imprudence."
Parliament speaker Ali Larijani also condemned Nimr's death, saying Riyadh would not emerge "easily from the quagmire they created by the martyrdom of this great sheikh."
All Iran's seminaries will be closed Sunday to protest Nimr's execution, with a demonstration expected in the Grand Mosque of Qom, the heart of Shiite faith in Iran, the ISNA news agency said.
The Basij student militia connected to Iran's elite Revolutionary Guards called for a demonstration Sunday afternoon in front of the Saudi embassy in Tehran.
Shiite Iran and Sunni Saudi Arabia are major rivals in the Middle East and are arrayed on opposing sides in several regional conflicts.
Nimr was a driving force of the protests that broke out in 2011 in Eastern Province, where the Shiite minority of Saudi Arabia complains of marginalization.
He was arrested in 2012, with the Interior Ministry describing him as an "instigator of sedition".