Iranian Supreme Leader vows 'divine vengeance' on Saudi Arabian politicians after execution of Nimr al-Nimr

'The unjustly spilled blood of this oppressed martyr will no doubt soon show its effect'

Iran's Supreme Leader has warned Saudi Arabian politicians they will face "divine vengeance" following the execution of a prominent Shia cleric.

"The unjustly spilled blood of this oppressed martyr will no doubt soon show its effect and divine vengeance will befall Saudi politicians," Iranian state TV reported Ayatollah Ali Khamenei as saying. It said he described the execution as a "political error".

Saudi Arabia executed Nimr al-Nimr and three other Shia alongside dozens of al-Qaeda members on Saturday, signalling it would not tolerate attacks by either Sunni jihadists or members of the Shia minority seeking equality.

Khamenei added: "This oppressed cleric did not encourage people to join an armed movement, nor did he engage in secret plotting, and he only voiced public criticism ... based on religious fervour."

The statement accused Tehran of "blind sectarianism" and said that "by its defense of terrorist acts" Iran is a "partner in their crimes in the entire region."

The comments came hours after Iranian protesters stormed the Saudi Embassy in Tehran early on Sunday.

Photos and video posted to social media, which have not been verified, show numerous protesters within the embassy compound, and fires lighting up the building. One video appears to show a Molotov cocktail being thrown at the building.

An Iranian journalist seemingly at the scene, Sobhan Hassanvand, posted images of masked protesters clutching a Saudi flag which appears to have been torn down, and video of protesters trashing rooms.

In an apparent swipe at Saudi Arabia's Western allies, Khamenei criticised "the silence of the supposed backers of freedom, democracy and human rights" over the execution.

"Why are those who claim to support human rights quiet? Why do those who claim to back freedom and democracy support this (Saudi) government?" Khamenei was quoted as saying.

While Western human rights groups have condemned the executions, Western government responses have so far been muted.

The US State Department expressed concern that Nimr's execution could exacerbate sectarian tensions in the Middle East. In Hawaii, where President Barack Obama is on vacation with his family, White House deputy national security adviser Ben Rhodes said the administration has urged the Saudis to show restraint regarding respect for human rights.

Khamanei's statement follow other highly charged comments by leading Iranian officials.

“The Saudi government supports terrorist movements and takfiri [ideology],” Iranian Foreign Ministry spokeman Hossein Jaber Ansari stated, “but confronts domestic critics with oppression and execution... the Saudi government will pay a high price for following these policies.”

There were small protests outside the Saudi embassy in London. Hilary Benn, shadow foreign secretary, tweeted: “Saudi Arabia profoundly wrong to execute Sheikh Nimr al-Nimr. Opposed to the death penalty and Amnesty had serious concerns about his trial.”

Protests were also reported in Yemen, Pakistan and Kahmir.

In Bahrain, which has a Shia majority and has long complained of persecution from Sunni Saudi Arabia, protests against the execution turned violent and saw police clash with protesters.

In Lebanon, Shia militant group Hezbollah denounced the execution as an “assassination” and the Lebanese Shia council called it a “grave mistake”.

In Iraq, there has been widespread condemnation. Moqtada al-Sadr, a powerful Shia leader who fought against the American occupation, called for "angry demonstrations” in protest.

Additional reporting by Reuters