Britain's ambassador to Iran was briefly detained on Saturday in Tehran after reportedly being caught up in protests that followed the government's admission that it accidentally shot down a Ukrainian passenger jet killing 176 people, most of them Iranian citizens.

Photos: CGTN

Iranian President Hassan Rouhani said a military probe into Wednesday's incident had found "missiles fired due to human error" brought down the Boeing 737, calling it an "unforgivable mistake."

How has Britain reacted?

Ambassador Robert Macaire was held for around an hour, according to reports, for allegedly "inciting" protesters in Tehran angry at the Iranian military's accidental downing of the jet.

Dominic Raab, Britain's foreign secretary, described the arrest as "a flagrant violation of international law" in a statement.

He warned Iran that it was "at a cross-roads moment," and had to choose between "its march towards pariah status" or "take steps to deescalate tensions and engage in a diplomatic path forwards."

Trump calls protests 'inspiring'

The protests in Iran were welcomed by U.S. President Donald Trump, who tweeted "we are following your protests closely, and are inspired by your courage."

Trump warned Tehran against cracking down on protests. "The government of Iran must allow human rights groups to monitor and report facts from the ground on the ongoing protests by the Iranian people," he tweeted.

His message came after Iranian police dispersed students chanting "radical" slogans, according to Fars news agency, during a Saturday gathering in Tehran to honor those killed when the Ukraine International Airlines plane was shot down shortly after taking off from Tehran.

AFP correspondents said hundreds of students had gathered early in the evening at Amir Kabir University, in downtown Tehran, to pay respects to those killed in the air disaster. The tribute later turned into an angry demonstration.