Saudi Arabia's execution Saturday of a prominent Shiite 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 marginalisation.
He was arrested in 2012, with the interior ministry describing him as an "instigator of sedition".
- 'Serious consequences' -
His execution also drew fire in Iraq, where the head of the Shiite Dawa party's parliamentary bloc urged Baghdad to close the Saudi embassy, expel the ambassador and "execute all Saudi terrorists in Iraqi prisons."
Khalaf Abdelsamad also warned the execution would have "serious consequences and bring about the end of the Al-Saud (ruling family's) rule."
Iraq's Prime Minister Haider al-Abadi and his predecessor Nuri al-Maliki both belong to the Dawa party.
In the holy Shiite city of Karbala, hundreds demonstrated and urged the closure of the newly opened Saudi embassy.
"I swear to God that the sheikh's blood was not spilled in vain," said Said Saad al-Mussawi, one of the protesters.
"We will make the earth shake under your feet," he said, addressing Saudi's royal family.
In Lebanon, the powerful Shiite movement Hezbollah accused Riyadh of "assassinating" Nimr.
It accused Washington of "direct and moral responsibility for this crime" because of its longstanding alliance with Riyadh.
Meanwhile, Syrian Information Minister Omran Zohbi described the executions as a crime, saying they were "an assassination of freedoms and human rights."
He urged international organisations to hold Riyadh accountable, saying "this group assassination is a horrific humanitarian and political crime."
Rights group Amnesty International said it appeared the kingdom was "using execution to settle political scores".
Middle East and north Africa director Philip Luther told AFP the kingdom was using "the guise of counter-terrorism" to clamp down on dissent.
The trials of Nimr and the others "were politicised on the one hand and grossly unfair, because the international standards for fair trial were grossly flouted.
"What is going on is an attempt to silence criticism of Saudi Arabia particularly among the Shia activist community".
The United Arab Emirates and Bahrain, members of a Saudi-led coalition battling Iran-backed Shiite rebels in Yemen, praised the executions.
UAE Foreign Minister Sheikh Abdullah bin Zayed al-Nahyan called them a "clear message against terrorism and those who call for and incite sedition and unrest to tear apart the society's unity and threaten social peace in the kingdom".
Bahrain, which has itself faced unrest from its Shiite majority population, also backed Riyadh in "all deterrent and needed measures it takes to confront violence and extremism".
Police used tear gas to disperse small demonstrations condemning the executions.