Tehran, Iran: Iran's supreme leader said Sunday that Saudi Arabia will face "divine revenge" for executing a top Shiite cleric whose death sparked protests in which the kingdom's embassy in Tehran was firebombed.

Ayatollah Ali Khamenei's remarks underlined the fury felt in Iran and other regional countries over the killing on Saturday of Nimr al-Nimr, who spent more than a decade studying theology in the Islamic republic.

Top officials in Iraq, Lebanon and Syria condemned the execution of Nimr, a force behind anti-government protests in Saudi Arabia in 2011 in the east of the country.

The 56-year-old cleric was put to death along with 46 Shiite activists and Sunnis who the Saudi interior ministry said had been involved in Al-Qaeda killings.

Some were beheaded, and others were shot by firing squad, said the ministry.

While Shiite leaders hit out at the executions the United Arab Emirates and Bahrain defended their Sunni ally, saying they were necessary to confront extremism.

Saudi Arabia in turn accused Iran of sponsoring terror and undermining regional stability.

Khamenei called the killing of Nimr "a political mistake by the Saudi government" which would "haunt its politicians". His comments came ahead of protests planned to start in Tehran at 3:00 pm (1200 GMT).

"The unjustly spilt blood of this martyr will have quick consequences," Khamenei told clerics in the Iranian capital. "God will not forgive."

"This scholar neither encouraged people into armed action nor secretly conspired for plots but the only thing he did was utter public criticism rising from his religious zeal," he said of Nimr.

The executions prompted protests in at least one city in Saudi Arabia's oil-rich Eastern Province, where Shiites complain of marginalisation, as well as in Iraq and Bahrain.

In Tehran the Saudi embassy was ransacked after protesters threw petrol bombs and stormed the building. The kingdom's consulate in Mashhad, Iran's second biggest city, was also set on fire.

Saudi foreign ministry spokesman Mansur al-Turki called Iran's reaction "irresponsible", and summoned Tehran's envoy in protest.

The embassy demonstrators were cleared out by police and 40 arrests have been made, Tehran's prosecutor told the ISNA news agency, adding that more detentions could follow.

"The fire has destroyed the interior of the embassy," an eyewitness told AFP.

- 'Exacerbating sectarian tensions' -

Websites had carried pictures of protesters clutching the Saudi flag, which had been pulled down and members of the crowd were able to climb onto the roof of the embassy before they were made to leave.

The incidents came after the United States and European Union expressed alarm over the executions, with Washington warning Riyadh "risked "exacerbating sectarian tensions at a time when they urgently need to be reduced".

Saudi Arabia's interior ministry said the executed men were convicted of adopting the radical "takfiri" ideology, joining "terrorist organisations" and implementing various "criminal plots".

An official published list included Sunnis convicted of involvement in Al-Qaeda attacks that killed dozens -- 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.

Those executed were Saudis, except for an Egyptian and a Chadian.

Elsewhere in the region, other Shiite countries and groups also reacted angrily.

In Saudi ally Bahrain, police used tear gas to disperse dozens of youths from the majority Shiite population protesting against the executions.

In Iraq, hundreds demonstrated in the holy Shiite city of Karbala and prominent Shiite lawmaker Khalaf Abdelsamad urged the closure of Riyadh's newly-reopened embassy in Baghdad and the expulsion of its ambassador.

- 'Instigator of sedition' -

Lebanon's Shiite movement Hezbollah, an ally of Tehran, said Saudi Arabia's rulers were "global criminals" and it denounced Nimr's execution as a "heinous crime".

In Yemen, where the kingdom is leading a coalition against Shiite rebels, the religious scholars' association controlled by them condemned the execution.

Nimr's brother, Mohammed, said he had hoped that "wisdom and a political solution" would prevail to spare the cleric's life.

Executions have soared in Saudi Arabia since King Salman ascended the throne a year ago -- 153 people put to death in 2015, nearly twice as many as in 2014.

Rights watchdogs have repeatedly raised concern about the fairness of trials in the kingdom, where murder, drug trafficking, armed robbery, rape and apostasy are all punishable by death.

Nimr was arrested in 2012, three years after calling for Eastern Province's Shiite-populated Qatif and Al-Ihsaa governorates to be separated from Saudi Arabia and united with Bahrain.

The interior ministry had described him at the time of his arrest as an "instigator of sedition".

A video on YouTube in 2012 showed Nimr making a speech celebrating the death of then-interior minister Prince Nayef bin Abdulaziz.