The UK's ambassador to Tehran has been arrested during protests outside a university, according to local reports.
Rob Macaire was detained for more than an hour before being released, the Iranian Tasnim news agency reported.
He was said to have been one of a number of individuals held on suspicion of organising, provoking and directing radical actions.
Mr Macaire was said to be "safe and well" in the UK embassy on Saturday night follwoing a detention of more than an hour in custody.
Foreign Secretary Dominic Raab condemned the arrest of Mr Macaire as a “flagrant violation” of international law.
Mr Raab said in a statement: “The arrest of our ambassador in Tehran without grounds or explanation is a flagrant violation of international law.
“The Iranian government is at a crossroads moment."
Mr Raab added: "It can continue its march towards pariah status with all the political and economic isolation that entails, or take steps to de-escalate tensions and engage in a diplomatic path forwards.”
Mr Macaire was said to be "safe and well" in the British embassy after he was eventually released after more than an hour in custody.
His arrest came as a wave of anti-government demonstrations broke out across the country following the admission Iranian forces had accidentally brought down a Ukrainian airliner killing all 176 people on board.
It is understood that the ambassador had been attending what had been billed as a vigil for the victims of the crash at the Amir Kabir University.
However the event quickly turned into an anti-government protest at which point he was said to have left.
It was as he was trying to make his way back to the embassy that he was picked up - although it is not clear by whom.
According to Iran's Tasnim news agency he was one of a number of people arrested outside the university on suspicion of organising, provoking, and directing radical actions.