Dozens of young Iranian men have entered buildings inside the British embassy compound in Tehran, throwing rocks, petrol bombs and burning documents looted from the offices.
The semi-official Fars news agency said security forces were trying to eject the protesters, who were a minority from a larger group staging an anti-UK demonstration outside the compound.
The agency referred to the protesters as students who were chanting
"Death to America", "Death to England" and "Death to Israel" among other
Live TV pictures showed several of the people on top of the entry gate to the embassy, waving Islamic flags.
One protester was seen waving a framed picture of Britain's Queen Elizabeth II apparently found inside the compound.
The British foreign ministry issued a statement saying it was "outraged" by the situation.
"It is utterly unacceptable and we condemn it," it said.
The Reuters news agency reported that embassy employees had fled the compound via a rear entrance and that Iranian riot police were trying to restore order.