Video report by ITV News Senior International Correspondent John Irvine
The UK's ambassador to Tehran has been arrested amid protests in Iran after the country admitted to "unintentionally" shooting down a passenger plane, killing all 176 on board.
Foreign Secretary Dominic Raab said the detention of Mr Macaire for more than an hour, without any grounds or explanation was a "flagrant violation" of international law.
Mr Macaire was said to have been one of a number of individuals held on suspicion of organising, provoking and directing radical actions, however it is understood he had been attending what was billed as a vigil for the victims of the crash.
The event quickly turned into a demonstration, at which point Mr Macaire is understood to have left.
Rob Macaire tweeted on Sunday to confirm he was not taking part in any demonstrations and was detained half an hour after leaving a vigil for the 176 victims.
US secretary of state Mike Pompeo tweeted images of the protests, claiming they demonstrated anger felt among Iranians toward their government.
"The voice of the Iranian people is clear," he wrote. "They are fed up with the regime’s lies, corruption, ineptitude, and brutality of the IRGC under Ayatollah Ali Khamenei's kleptocracy.
"We stand with the Iranian people who deserve a better future."
Earlier on Saturday Iran admitted its military "unintentionally" shot down the Ukraine passenger plane.
The Iranian government had previously denied Western accusations they were responsible however, in the face of mounting evidence it acknowledged it had it shot down the Ukrainian jetliner by accident.
Following the admission hundreds of furious protesters gathered at universities in Tehran as disdain grew against Supreme Leader Ayatollah Ali Khamenei.
Demonstrators demanded officials involved in the missile attack be removed from their positions and arrested.
In a statement, Foreign Secretary Dominic Raab said the arrest of the UK ambassador in Tehran "without grounds or explanation is a flagrant violation of international law".
"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."