The UK ambassador to Iran was arrested during anti-government protests in Tehran after officials admitted shooting down a passenger plane.
Rob Macaire was held for three hours, in what the UK described as a “flagrant violation of international law”.
It is understood he was attending a vigil for those who died in the crash, which turned into a protest.
He left to make his way back to the UK embassy, but was arrested as he stopped at a barber shop for a haircut.
In a statement, UK Foreign Secretary Dominic Raab 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.”
The Iranian Etemad newspaper shared a picture of the ambassador on Twitter after the Tasnim news agency reported his arrest.