The British ambassador to Iran was arrested during protests against the regime and detained briefly in the capital Tehran, said the UK Foreign & Commonwealth Office.
Semi-official Iranian news agency Tasnim reported that Robert Macaire, 53, was held on Saturday for his presence at a gathering in front of Amirkabir University.
Mr Macaire was released a few hours later, Tasnim said.
UK Foreign Secretary Dominic Raab said: “The arrest of our ambassador in Tehran without grounds or explanation is 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 de-escalate tensions and engage in a diplomatic path forwards.”
Mr Macaire wrote on Twitter: "Thanks for the many goodwill messages. Can confirm I wasn’t taking part in any demonstrations! Went to an event advertised as a vigil for victims of PS752 tragedy. Normal to want to pay respects- some of victims were British. I left after 5 mins, when some started chanting."
He said he was detained half an hour after leaving the area.
Iran's deputy foreign minister said Mr Macaire, was arrested as a foreigner at "an illegal gathering" but was freed soon after being identified.
"He wasn't detained, but arrested as unknown foreigner in an illegal gathering," Seyed Abbas Araghchi tweeted, adding Mr Macaire was released 15 minutes after he called the British diplomat to confirm his identity.
Mr Macaire has been summoned to the Iranian foreign ministry over his "unconventional conduct".
The US called for Iran to apologise for detaining the ambassador.
"This violates the Vienna Convention, which the regime has a notorious history of violating. We call on the regime to formally apologise to the UK for violating his rights and to respect the rights of all diplomats," State Department spokeswoman Morgan Ortagus tweeted.
The Iranian regime arrested the British ambassador to Iran. This violates the Vienna Convention, which the regime has a notorious history of violating. We call on the regime to formally apologize to the UK for violating his rights and to respect the rights of all diplomats.
— Ned Price (@StateDeptSpox) January 11, 2020
The move was also condemned by the German Foreign Office, who said on Twitter: "Temporary detention of the UK Ambassador to Tehran by Iranian security forces is a completely unacceptable violation of international law. We strongly condemn this action. Compliance with the most elementary rules of international conduct is in the interest of all."
Some Iranian MPs, however, have demanded Mr Macaire be expelled from Iran according to Fars News.
Hojatoleslam Alireza Salimi said the ambassador's presence at the 'protests' was "strange" as he urged the Iranian government to take action.
Abbas Goodarzi accused Mr Macaire of "interfering in Iran's internal affairs".
Protests have erupted in Tehran after Iran admitted to accidentally shooting down a Ukrainian airliner, killing all 179 people on board.
US President Donald Trump said on Saturday that he was following the protests closely.
“To the brave, long-suffering people of Iran: I’ve stood with you since the beginning of my Presidency, and my Administration will continue to stand with you,” he said in separate Twitter posts in English and Farsi.
“We are following your protests closely, and are inspired by your courage.”
