A breakaway group of demonstrators has stormed the British Embassy compound in Tehran.
Dozens of young Iranian men entered the British Embassy compound, throwing rocks and petrol bombs and burning the British flag along with documents looted from offices, Al Jazeera reported.
The trouble-makers were a breakaway group from larger anti-UK demonstrations staged outside the embassy to protest against Britain's imposition of new sanctions on the Islamic state in response to its alleged nuclear programme.
Protesters shouted "Death to England", "Death to America" and "Death to Israel" as they hoisted an Iranian flag in place of the Union Jack.
Iran's semi-official Mehr News Agency said documents from the embassy had been seized and set alight while embassy staff fled by the back door.
The UK'S Foreign Office has condemned the attack as "unacceptable" and called on the Iranian government to "act urgently to bring the situation under control".
Iranians are angry about what they see as an unjustified tightening of sanctions against their country. Britain, Canada and the United States, urged by Israel, last week announced they would impose stricter financial constraints on the Islamic state in the wake of a report published by the International Atomic Energy Agency (IAEA), which suggested that Tehran might soon have the capabilities to build a nuclear warhead.
Iran insists its nuclear enrichment facilities are for civilian use only.
Critics say that the IAEA report has proved "nothing at all" and is merely being used as justification to increase the international pressure on Iran. Israel's Prime Minister, Binyamin Netanyahu, has hinted at the possibility of military action against Iran, a scenario that could lead to full-scale war in the Middle East.
Iran's parliament voted to downgrade diplomatic relations with Britain as a result of the sanctions.
Protesters also stormed a second British Embassy compound in northern Tehran, The Daily Telegraph reported. The situation remains volatile.