The UK's ambassador to Iran was arrested during protests in Tehran after the country admitted that it shot down a passenger plane by mistake.
Rob Macaire was held for more than an hour, according to local media.
The Foreign Office described his arrest as a "flagrant violation of international law".
Earlier, the Prime Minister said Iran's admission was an "important first step" and called for an investigation into the "tragic accident".
Iran has now admitted Ukraine International Airlines flight PS752 was shot down due to "human error".
The Iranian Etemad newspaper shared a picture of the ambassador on Twitter after the Tasnim news agency reported his arrest during the protests outside a university.
Iranian media claimed Mr. Macaire was accused of inciting anti-government protests.
In a statement issued by Downing Street, 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 deescalate tensions and engage in a diplomatic path forward."
There were demonstrations in Tehran and other cities on Saturday as people gathered to commemorate those who died in the downing of the Ukrainian plane.
Missile strikes
Iran initially denied responsibility for Wednesday's crash - which killed 176 people, including four Britons.
But on Saturday, President Hassan Rouhani admitted the Iranian military had "unintentionally" shot down the passenger plane after mistaking it for a cruise missile when it turned towards a sensitive military site.
President Rouhani said the missile strike was an "unforgivable mistake".
The crash came just hours after Iran carried out missile strikes on two airbases housing US forces in Iraq.
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