Chanting slogans including "Death to England,” the protesters, msot of them students, attacked the embassy compound, removed the mission's flag, replacing it with Iran’s, and ransacked its offices. They also threw stones and Molotov cocktails at embassy buildings, causing a fire in one of them.

Police entered the compound about half an hour after the attack began and managed to remove some of the protesters from its territory. Several dozen attackers are still believed to be inside the embassy.

The Iranian Mehr news agency reported on its website that six people had been taken hostage in the embassy, but then deleted the report that has not been officially confirmed. The Fars news agency later said the embassy employees were released.

The British government said it was "outraged" by the incident. A Foreign Office spokesman called on the Iranian authorities to "act urgently to bring the situation under control.”

The Russian Foreign Ministry condemned the attack in a statement and called on Iranian authorities to provide security for all foreign diplomatic missions.

“Such actions by a crowd of protesters committed in breach of generally accepted norms of international law, which guarantees the inviolability of territory and property of diplomatic missions, are inadmissible and must be condemned,” the statement reads.

“We hope that the Iranian authorities will take all measures necessary to immediately restore order, investigate the incident and prevent such events from taking place in the future,” the Foreign Ministry said.

The attack was part of a larger demonstration that followed a decision by the Iranian authorities to expel the British ambassador from the country within 15 days. The relevant parliamentary bill was approved by Iranian lawmakers on Monday after Britain announced in mid-November that it was cutting off all ties with the Islamic Republic’s financial sector.

The decision was part of unilateral sanctions imposed on Iran by Britain, the United States and Canada following the publication of an International Atomic Energy Agency (IAEA) report on Iran’s nuclear program earlier in November. According to the report, Iran worked on developing a nuclear bomb at least up until last year.

Tehran denies the Western allegations that its nuclear program is aimed at building weapons, saying it needs nuclear energy to generate electricity.