Earlier today, Saudi Arabia announced it had staged its largest mass execution in 25 years.

43 al-Qaeda conspirators were killed along with 4 Shiites accused of shooting policemen in the anti-government protests which broke out during the Arab Spring. Among the Shiites killed: prominent cleric Nimr al-Nimr.

His death drew sharp criticism from Iran and Hezbollah with the latter calling the execution a “grave mistake.” Here's WSJ:

Lebanese militant group Hezbollah called Mr. al-Nemer’s execution a “stain that would haunt this regime,” while former Iraqi Prime Minister Nouri al-Malki, another ally of Iran, said the execution “will topple the Saudi regime.” Arab Gulf allies of the kingdom such as Bahrain and the United Arab Emirates said they supported steps taken by Saudi Arabia to “confront terrorism.” Mr. al-Nemer was sentenced to death in October 2014 and charged with crimes including disobeying the ruler, inciting sectarian strife and bearing arms against security forces. Mr. al-Nemer wasn’t widely known outside Qatif before 2011, when he emerged as a leading voice behind Shiite protests that rocked the oil-rich eastern part of Saudi Arabia for two years. He was arrested after a car chase near his family’s farm in their hometown of Awwamiya in July 2012. Authorities said he opened fire at security forces. His family has denied this and said he was unarmed.

“We condemn a deplore this unjust killing and consider it an example of killing wisdom and moderation,” Mr. al-Nemer’s family said in a statement.

"The Saudi government supports terrorists and takfiri [heretic] extremists, while executing and suppressing critics inside the country," Iran's Foreign Ministry declared. These are "hostile statements," Riyadh responded, adding that Iran's comments constitute "a blatant intervention in the kingdom’s affairs.”

Protests erupted in the Qatif district of Saudi Arabia's Eastern Province as well as in Bahrain, where hundreds took to the streets, burning tires and braving tear gas fired by police. As we reported earlier today, protesters had also converged on the Saudi embassy in Iran.

Now, in what looks like a repeat of the Iran Hostage Crisis, the protesters in Tehran have reportedly broken into the Saudi embassy and set it ablaze with Molotov cocktails.

"Images shared on social media early on Sunday morning appeared to show Iranian protesters breaking into Saudi Arabia's embassy in Tehran and starting fires, after gathering there to denounce the kingdom's execution of a Shi'ite cleric," Reuters reports. "One photograph, posted on Twitter, showed protesters outside the embassy building with small fires burning inside, while another showed a room with smashed furniture purportedly inside the building."

Video shows protesters inside Saudi embassy in Tehran pic.twitter.com/DEmsNLI6ZG — Sobhan Hassanvand (@Hassanvand) January 2, 2016

It's 23:50 here in Tehran, seems protest agnst Nimr execution continues outside Saudi embassy in Tehran right now pic.twitter.com/5LLj89FhYn — Sobhan Hassanvand (@Hassanvand) January 2, 2016

#BREAKING: Angry protesters storm Saudi embassy in Tehran pic.twitter.com/xOX5n6QtsR — Sobhan Hassanvand (@Hassanvand) January 2, 2016

Protesters break into Saudi embassy building in Tehran pic.twitter.com/7wtBGpZuco — Sobhan Hassanvand (@Hassanvand) January 2, 2016

Police disperse protesters at Saudi embassy in Tehran Firefighters are trying to put off fire at embassy pic.twitter.com/WWb1QCURQG — Sobhan Hassanvand (@Hassanvand) January 2, 2016

Saudi Arabia flag brought down at embassy in Tehran pic.twitter.com/7ptL0gV0ZF — Sobhan Hassanvand (@Hassanvand) January 2, 2016

Protesters in Mashhad bring down Saudi flag at consulate following al-Nimr execution pic.twitter.com/2abn4V2KgW — Sobhan Hassanvand (@Hassanvand) January 2, 2016

VIDEO: Footage from earlier tonight of protesters throwing Molotov cocktails at Saudi embassy in Tehran - @hamid3663 pic.twitter.com/j6ATZDYEzZ — Conflict News (@Conflicts) January 2, 2016

As a reminder, the Sheikh was a prominent voice in the anti-government movement in Saudi Arabia. Riyadh effectively killed (literally) two birds with one stone with his execution: 1) a dissident voice was forever silenced, 2) a message was sent to Tehran, whose regional influence is expanding in Iraq and Yemen and whose forces have served to shore up the Assad government in its fight against Sunni extremists in Syria.

Now, the Iranians look set to send a message back to Riyadh as the sectarian divide once again rears its ugly head.

Meanwhile, Iran's foreign ministry is calling for "calm." Here's Reuters:

Iran's Foreign Ministry called for calm in the early hours of Sunday after police dispersed angry protesters who had stormed Saudi Arabia's embassy in Tehran. Demonstrators broke into the embassy and started fires before being cleared away by the police, Iran's ISNA news agency reported, after gathering there to denounce the kingdom for executing a prominent Shi'ite cleric on Saturday. The ministry issued a statement calling on protesters to respect the diplomatic premises, according to the Entekhab news website.

If this situation escalates, clearly crude oil will be well bid on Sunday afternoon.