The Saudi embassy in Tehran has come under attack following a public backlash against the execution of Shia cleric Nimr al Nimr. Dramatic video footage has emerged on social media showing protestors inside the Embassy, smashing windows and ransacking the building, as they express their profound sense of outrage at the brutal killings of 47 detainees.

Petrol bombs were thrown by protestors and pictures posted on social media show the building ablaze.

Saudi Arabia has faced global condemnation after it was revealed on 2 January, that 47 people at prisons around the country, including 56-year-old al Nimr, were executed. Most of the detainees had been captured after a series of attacks by al Qaeda between 2003 and 2006. Mr Nimr, who was a driving force behind the anti-government protests, was found guilty of a number of terrorism-related charges in 2014, including incitement of vandalism and sectarian strife, failing to obey or pledge allegiance to King Abdullah (then monarch of Saudi Arabia), calling for the collapse of the state, and insulting relatives and companions of the Prophet Muhammad.

The executions took place in 12 cities in Saudi Arabia, with four prisons carrying out the sentences by firing squads and the others beheading. In a statement, Saudi Arabia's Interior Ministry attempted to justify the barbaric executions, claiming that the 47 had been convicted of adopting the radical "takfiri" ideology, joining "terrorist organisations" and implementing various "criminal plots".

News of the executions sparked unrest in neighbouring Bahrain and in the eastern Iranian city of Mashhad, where demonstrators have already taken to the streets. Troops are reportedly moving into Saudi areas where the Shia minority was due to protest Sky News reports.

Criticism of Mr Nimr's execution has been led by Shia-dominated Iran, Saudi Arabia's main rival in the Middle East. The Foreign Ministry warned the kingdom would "pay a high price", while a leading Iranian cleric, Ayatollah Ahmad Khatami, predicted the repercussions will "wipe" the Saudi ruling family "from the pages of history".

Former Iraqi PM Nouri al Maliki said in a statement Mr Nimr's death "will topple the Saudi regime". The Lebanese militant group Hezbollah called it an "assassination" and the country's Supreme Islamic Shia Council described it as a "grave mistake".

Protests were also reported on Saturday in Yemen, Pakistan and Indian-administered Kashmir. The US said it was concerned the execution "risks exacerbating sectarian tensions at a time when they urgently need to be reduced".

In the UK, Shadow foreign secretary Hilary Benn said carrying out the death sentence on Mr Nimr was "profoundly wrong". A demonstration was held outside the Saudi embassy in London in protest at the executions.

Shane Enright of Amnesty International UK said the death penalty was "unacceptable in all circumstances" and was particularly concerned that a number of "peaceful dissidents" had been killed, including Mr Nimr. The international rights group said the 47 executions demonstrated the Saudi authorities' "utter disregard for human rights and life" and called Sheikh Nimr's trial "political and grossly unfair".

The international human rights group Reprieve said today's actions showed the Saudi government was "continuing to target those who have called for domestic reform in the kingdom" and the organisation has urged the European Union to intervene with Saudi Arabia to prevent further killings.

Maya Foa, director of the death penalty team at Reprieve, said news of the killings sparks fears that more executions are imminent. "Today's appalling news, with nearly 50 executed in a single day, suggests 2016 could be even worse. Alarmingly, the Saudi government is continuing to target those who have called for domestic reform in the kingdom, executing at least four of them today.

"There are now real concerns that those protesters sentenced to death as children could be next in line to face the swordsman's blade. Saudi Arabia's allies - including the US and UK - must not turn a blind eye to such atrocities and must urgently appeal to the kingdom to change course."

The list of those executed did not include Mr Nimr's nephew, Ali al Nimr, who was 17 when he was arrested in 2012. However fears, are growing for Ali, who was sentenced to death by crucifixion for alleged participation in the anti-government protests. Arrested in 2012, his sentence has sparked worldwide outcry from human rights groups. Activists and campaigners allege al-Nimr's conviction is politically motivated as he is the nephew of Sheikh Nimr al-Nimr.

Initial reports that he had been sentenced to death sparked global outrage prompting Labour leader Jeremy Corbyn to write to Prime Minister David Cameron urging him to intervene. Liberal Democrat leader Tim Farron responded to news of the executions by describing capital punishment as "abhorrent" and expressed his concerns for Ali Mohammed al-Nimr.

"With the carrying out of this large number of executions there will now be huge international concern about what will happen to Ali Mohammed al-Nimr who is Sheikh al-Nimr's nephew. He was just 17 years old when he was sentenced to death. The Foreign Secretary has told Parliament that he does not expect Ali Mohammed al-Nimr to be executed but he now needs to seek fresh assurances that he will be reprieved."

Mr Nimr's brother has called for a "peaceful" response to the executions, saying the family did not want to see further bloodshed. He told Reuters: "We hope that any reactions would be confined to a peaceful framework. No one should have any reaction outside this peaceful framework. Enough bloodshed."

The Saudi state said that out of the 47 executed on New Year's Day, one was a Chadian national while another was Egyptian. The rest were Saudis, including Sunnis convicted of terror attacks. Last year 157 people were put to death in Saudi Arabia, compared to 90 in 2014.

A Facebook page has been created to rally support in protest at the death sentence on Ali Mohammed al-Nimr.