Iran's supreme leader warned on Sunday that Saudi Arabia would face "divine revenge" for executing a Shiite cleric as condemnation also poured in from Iraq and protesters took to the streets.
Ayatollah Ali Khamenei was joined in his condemnation of Sheikh Nimr al-Nimr's death by Iraq's top Shiite authority, Grand Ayatollah Ali al-Sistani, who called the death sentence an unjust act of aggression.
Their comments came as protests in Iran on Sunday spread to Bahrain, Pakistan, Indian Kashmir and Lebanon a day after a mob set fire to the Saudi embassy in Tehran and ransacked it before dozens were arrested.
On top of the ongoing wars in Syria and Yemen, Nimr's execution set the Middle East's main Shiite countries further apart from their Sunni counterpart in Riyadh.
Nimr, a Saudi Shiite who spent more than a decade studying theology at Iran's seminaries, was a force behind anti-government protests in eastern Saudi Arabia in 2011.
He was put to death Saturday along with 46 Shiite activists and Sunnis who Saudi Arabia said were involved in Al Qaeda killings.
Some were beheaded, others were shot by firing squad.
The 56-year-old's execution also sparked concern in the United Nations, the European Union and the United States and was deplored by Germany and France.
Britain, which is careful to protect deep trade and investment links with Saudi Arabia, reiterated its opposition to the death penalty in a statement which avoided mentioning Nimr directly.
Saudi Arabia's Gulf partners defended their ally, with the United Arab Emirates, Bahrain, Qatar, Kuwait and Yemen calling the executions Riyadh's sovereign right to confront extremism.'God Will Not Forgive'
Khamenei said the killing would not go unanswered.
"The unjustly spilt blood of this martyr will have quick consequences," he told clerics in Tehran.
"God will not forgive... it will haunt the politicians of this regime."
Iraqi religious leaders also reacted with outrage.
Grand Ayatollah Sistani called the execution "an injustice and an aggression" while another cleric, Mohammed Taqi al-Mudaressi, said it was a "declaration of war" against Shiites.
Iraq's foreign ministry accused Saudi Arabia of using the fight against "terrorism" to silence its opposition.
In Lebanon, the head of the Shiite Hezbollah movement allied to Iran, Hassan Nasrallah, accused Riyadh of seeking to spark a "conflict between Sunni and Shiite" Muslims.
Saudi Arabia had branded Nimr an "instigator of sedition" and arrested him in 2012, after a video on YouTube showed him making a speech celebrating the death of the then interior minister.
Three years earlier he had called for the oil-rich Eastern Province's Shiite-populated Qatif and Al-Ihsaa governorates to be separated from Saudi Arabia and united with Bahrain.
Shiites in the neighbouring countries complain of marginalisation.
Demonstrations outside the Saudi embassy and at Palestine Square in Tehran attracted around 1,500 people Sunday, with chants of "Death to the House of Saud".
"His death will start a revolution which hopefully will lead to the fall of the Saudi family," said Rezvan, a 26-year-old in a traditional black chador who declined to give her last name.
Iran's President Hassan Rouhani condemned Nimr's execution but also denounced attacks on the Saudi embassy in Tehran and the consulate in second city Mashhad.'The gates of hell'
He branded the demonstrators "radicals" and said the assaults were "totally unjustifiable". Forty-four people were arrested, prosecutors said.
Small protests took place in Iraq and in the Lebanese capital.
"The House of Saud has opened the gates of hell on its own regime," said one cleric, Ahmed al-Shahmani, on Baghdad's Palestine Street.
In Sunni-ruled Bahrain, police used buckshot and tear gas against Shiite protesters who threw petrol bombs. An interior ministry official said several people, including minors, were detained.
Concern mounted around the world, with the US warning that Riyadh risked "exacerbating sectarian tensions at a time when they urgently need to be reduced".
UN Secretary General Ban Ki-moon said he was "deeply dismayed" by the state-sanctioned killings, while France and Germany voiced concerns about growing tensions in the Middle East.
Saudi Arabia's interior ministry said the executed men were convicted of adopting the radical "takfiri" ideology, joining "terrorist organisations" and implementing "criminal plots".
They included Sunnis convicted of involvement in Al-Qaeda attacks that killed dozens of Saudis and foreigners in 2003 and 2004.
Among them was Fares al-Shuwail, described by Saudi media as Al-Qaeda's top religious leader in the kingdom.
Executions have soared in Saudi Arabia since King Salman ascended the throne a year ago with 153 people put to death in 2015, nearly twice as many as in 2014, for crimes ranging from murder to drug trafficking, armed robbery, rape and apostasy.
Human Rights Watch said Saturday's "mass execution was the largest since 1980" when 68 militants who had seized Mecca's Grand Mosque were beheaded.
"Saudi Arabia had a shameful start to 2016, executing 47 people in a day, after a year with one of the highest execution rates in its recent history," said HRW's Middle East director Sarah Leah Whitson.
Amnesty International said Saudi Arabia was using Nimr's execution "to settle political scores".