Iranian protesters stormed the Saudi Embassy in Tehran as Shi'ite Muslims reacted with fury to Saudi Arabia's execution of a prominent Shi'ite cleric.
Demonstrators who had massed at the embassy gates to protest at Nimr al-Nimr's execution broke into the embassy and started fires before being cleared away by the police.
Scenes were reminiscent of the storming of the US Embassy 37 years ago - Iranian demonstrators, this time ransacking the Saudi embassy and setting it on fire.
They were outraged at the execution of Sheikh Nimr al-Nimr, a cleric who was a voice for Saudi Arabia's Shia minority and according to followers, a peaceful reformist.
Nimr had been shot and arrested in 2012 during anti-government protests that erupted as part of the Arab spring.
He was one of 47 simultaneously put to death by Saudi Arabia in 12 locations yesterday after all were convicted of belonging to Al Qaeda.
It was the Sunni-led kingdom's largest mass execution since 1980.
It is so dangerous because of a sectarian divide between Muslims, in which Saudi Arabia is the main Sunni kingdom, and Iran, the main protector of Shia Islam.
Already Sunni Muslims are pitted against Shia Muslims in conflicts across Syria, Iraq and Yemen.
In Nimr's home province, Sunnis took to the streets in defiance but the demonstrations were peaceful.
In neighbouring Bahrain though there were scuffles as police fired tear gas.
And the demonstrations have spread as far as London where a group of protesters chanted slogans outside the Saudi Arabian embassy.
Back here, the Greens are calling for New Zealand to pull out of an impending free trade deal with the Saudi government.
But all eyes will be on Iran for an escalation.
Ominously, the Revolutionary Guard which was behind the storming of the American embassy in Tehran in 1979 has threatened to exact a "harsh revenge" for Nimr's execution.
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