Around 20 Iranian protesters on Tuesday stormed the British embassy in Tehran, removing the mission's flag and ransacking offices, an AFP journalist outside the compound reported.
The protesters were also shown live on Iranian state television throwing stones at embassy windows, breaking them, and one was seen climbing the wall with a looted portrait of Britain's Queen Elizabeth II.
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The intrusion occurred as ranks of Iranian police in riot gear stood by, doing nothing.
Outside the embassy's walls, several hundred other demonstrators were gathered, some of them chanting "Death to Britain" and demanding the British ambassador leave the country immediately.
The demonstration came a day after Iran passed a law to expel the ambassador within the next two weeks in retaliation for new British sanctions that cut off all ties with Iran's financial sector.
Britain has threatened to act "robustly" if Iran's foreign ministry follows through by kicking out its ambassador, Dominick Chilcott, who took up his post only last month.