The U.K. ambassador to Iran, Robert Macaire, was arrested and detained for more than an hour Saturday after attending a demonstration outside of a university in Tehran, Iranian state media reported. Macaire was held on suspicion of organizing, provoking, and directing radical actions, according to the Tasnim news agency. Macaire had been attending a vigil for the victims of the crash, which turned into a demonstration, The Guardian reports. Macaire left once the focus of the gathering changed, and he was reportedly arrested while returning to the British embassy. British Foreign secretary Dominic Raab said in a statement that Macaire’s arrest “without grounds or explanation is a flagrant violation of international law.”

“The Iranian government is at a crossroads moment,” Raab said. “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.”