Britain's Ambassador to Iran was arrested in Tehran on Saturday during protests outside a university, The Independent reported, citing the UK Foreign office.
British Foreign Secretary Dominic Raab said the arrest of the ambassador, Rob Macaire, was a "flagrant violation of international law".
According to the Iranian Tasnim news agency, Macaire was detained for more than an hour before being released.
He is reportedly one of a number of individuals held on suspicion of organizing, provoking and directing radical actions.
“The arrest of our ambassador in Tehran without grounds or explanation is a flagrant violation of international law,” Raab said, according to The Independent.
"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," he added.
Macaire had reportedly been attending what had been billed as a vigil for the victims of the crash of the Ukrainian plane earlier this week, but the event quickly turned into a demonstration at which point Macaire left.
It was as he was returning to the British embassy that he was arrested. Following a number of urgent calls to the Iranian Ministry of Foreign Affairs he was eventually released and allowed to return to the embassy, according to The Independent.
The incident took place as Iranians took to the streets of Tehran to demand Supreme Leader Ali Khamenei step down after the military shot down the civilian Ukrainian plane.
On Saturday, Iran finally admitted that it had made a mistake and shot the Ukrainian plane after it flew too close to a sensitive military site and failed to respond to signals.
In 2011, Britain cut its ties with Iran, after the British Embassy in Tehran was stormed by Iranian students who were protesting against Western sanctions over Iran's nuclear program.
Two years later, Britain resumed its ties with Iran by naming a charge d’affaires, a diplomatic post that is one level below ambassador.
In 2016, the two countries appointed new ambassadors to each other's countries. The move followed the signing of the Iranian nuclear deal, of which Britain is a part.
