The British ambassador to Iran has denied taking part in demonstrations after he was held by Iranian authorities during protests over a fatal plane crash.

Iranian authorities detained Rob Macaire on Saturday on suspicion of organizing, provoking and directing radical actions, Sky News reported.

It came as riot police fired tear gas at thousands of Iranians who had taken to the streets to direct their anger at Supreme Leader Ali Khamenei, hours after the country's Revolutionary Guard admitted shooting down a Ukrainian passenger plane near the capital.

The Iranian authorities had initially denied being responsible for bringing down Ukrainian International Airlines flight PS752 on Wednesday – but backtracked in the face of growing evidence to the contrary.

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Mr. Macaire tweeted in Farsi and in English: "Thanks for the many goodwill messages. Can confirm I wasn't taking part in any demonstrations! Went to an event advertised as a vigil for victims of #PS752 tragedy.

"Normal to want to pay respects – some of victims were British. I left after 5 mins, when some started chanting.

"Detained half an hour after leaving the area. Arresting diplomats is of course illegal, in all countries. See comments by Foreign Secretary @DominicRaab".

The largest share of the 176 people aboard the flight who died were Iranians, many with dual citizenship, 57 were Canadian and four were British.

On Saturday evening, Mr. Raab had issued a statement which said: "The arrest of our ambassador in Tehran without grounds or explanation is a flagrant violation of international law.

"The Iranian government is at a cross-roads moment. It can continue its march towards pariah status with all the political and economic isolation that entails, or take steps to deescalate tensions and engage in a diplomatic path forwards."

UK security minister Brandon Lewis told Sky's Sophy Ridge on Sunday that the government was calling on Iran to co-operate with investigations and to de-escalate tensions.

Earlier today, riot police and plainclothes officers massed in Tehran's Vali e Asr Square as calls circulated for further protests. A large black banner was unveiled in the square bearing names of those who died in the crash.

Overnight, Iranian protesters and newspapers piled pressure on the country's leadership.