Protesters in the Iranian capital, Tehran, have broken into the UK embassy compound during a demonstration against sanctions imposed by Britain.
Militant students are said to have removed the British flag, burnt it and replaced it with Iran’s flag. State TV showed youths smashing embassy windows.
The move comes after Iran resolved to reduce ties following the UK’s decision to impose further sanctions on it.
The UK’s Foreign Office said it was “outraged” by the actions.
It called on Iran to honour international commitments to protect diplomatic missions and their staff. A statement also urged Britons in Iran to “stay indoors and keep a low profile”.
The students clashed with riot police and chanted “the embassy of Britain should be taken over” and “death to England”, AP reports.
Students were reported to be ransacking offices inside the building, and one protester was said to be waving a framed picture of Queen Elizabeth II.
Iran’s semi-official Mehr news agency said embassy documents had been set alight. Embassy staff fled by the back door, the agency added.
Pictures showed a car inside the compound on fire while outside the embassy’s walls, several hundred other demonstrators were gathered.
Some two hours later, police seemed to be back in control of the building. Live TV footage showed riot police gradually clearing the protesters away from outside the embassy.
“The rally is ended, leave,” police called from loudspeakers.
An unconfirmed report from the official Irna news agency said a separate group of protesters had broken into another British embassy compound in the north of the city and seized “classified documents”.
The UK Foreign Office condemned the attack. “We are outraged by this. It is utterly unacceptable,” it said in a statement.
“The Iranian government have a clear duty to protect diplomats and embassies in their country and we expect them to act urgently to bring the situation under control and ensure the safety of our staff and security of our property”.
It later updated its travel advice to Iran, urging Britons there to “stay indoors, keep a low profile and await further advice”.
It was not clear how many embassy staff were in the building at the time. A Foreign Office source said it was checking on the well-being of workers and diplomats, AP reported.
France condemned the attack “very strongly”, French Foreign Minister Alain Juppe said.
“France expresses its full solidarity with the UK,” he said.
Last week the US, UK and Canada announced new measures targeting Iran over its controversial nuclear plans.
For its part, the UK Treasury imposed sanctions on Iranian banks, accusing them of facilitating the country’s nuclear programme
That decision followed a report from the International Atomic Energy Agency (IAEA) that suggested Iran was working towards acquiring a nuclear weapon.
It said Iran had carried out tests “relevant to the development of a nuclear device”.
Iran insists its nuclear programme is for peaceful purposes only.
On Sunday, Iran’s parliament voted by a large majority to downgrade diplomatic relations with the UK in response to the British action.
Iranian radio reported that some MPs had chanted “Death to Britain” during the vote, which was approved by 87% of MPs.