Britain's Foreign Office confirmed on Saturday that the country's ambassador in Tehran was briefly detained by Iranian authorities, denouncing the arrest as a "flagrant violation of international law".
Earlier, the Tehran-based Tasnim news agency reported that British envoy Rob Macaire was arrested for several hours in front of Amir Kabir University for inciting anti-government protesters.
"The arrest of our Ambassador in Tehran without grounds or explanation is a flagrant violation of international law," Foreign Secretary Dominic Raab said in a statement.
"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 de-escalate tensions and engage in a diplomatic path forwards," Raab said.