Britain’s ambassador to Iran, Rob Macaire, has been arrested and held for several hours after attending a vigil for the victims of the Ukraine plane crash which turned into a demonstration.

Details of his arrest were announced by the country’s semi-official Tasnim news agency and prompted an angry response from Foreign Secretary, Dominic Raab.

"The arrest of our Ambassador in Tehran without grounds or explanation is a flagrant violation of international law,” he said.

"The Iranian government is at a crossroads 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 forward."

It is understood that the event which Mr Macaire attended had been advertised as a vigil at Amir Kabir University for the 176 victims of the Tehran plane crash last week.

He and another member of the embassy staff left once the vigil turned into a protest.