Britain's ambassador to Tehran has said he was detained briefly by Iranian authorities as he attended a vigil for the victims of last week's crash of a Ukrainian passenger jet.
Iran's Mehr news agency said Rob Macaire was arrested on Saturday for his alleged "involvement in provoking suspicious acts" at the gathering in front of Tehran's Amir Kabir University.
Students held a gathering at the school after Iran said the Ukrainian airliner was downed by mistake by Iranian antiaircraft missiles.
In a post to Twitter Sunday, Macaire said he attended the event to pay respects to the victims, and was not attending any demonstration.
The British Foreign Ministry called Macaire's detention "a flagrant violation of international law."
"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 forwards," the ministry said.
Iranian officials did not immediately make any statement about the incident.
More protests were expected later on Sunday, amid building outrage among some Iranians about the downing of the Ukrainian jet.