TEHRAN: Iranian protesters stormed two British Embassy compounds in Tehran on Tuesday, smashing windows, hurling petrol bombs and burning the British flag in a protest against sanctions imposed by Britain, live Iranian television showed.

Iran's semi-official Mehr news agency said protesters took six British diplomatic staff hostage from an embassy compound in the north of the city but it withdrew the story from its website minutes later without giving any explanation.

The attacks followed the rapid approval by Iran's Guardian Council of a parliamentary bill compelling the government to expel the British ambassador in retaliation for the sanctions. A lawmaker had also warned on Sunday that angry Iranians could storm the British Embassy as they did the U.S. mission in 1979.

Several dozen protesters broke away from a crowd of a few hundred protesters outside the main embassy compound in downtown Tehran, scaled the embassy gates and went inside.

Mehr news agency said protesters pulled down the British flag, burned it, and put up the Iranian flag. Inside, the demonstrators threw stones and petrol bombs. One waved a framed picture of Queen Elizabeth, state TV showed. The British Foreign Office said it was outraged.

“There has been a incursion by a significant number of demonstrators into our Embassy premises, including vandalism to our property,” the Foreign Office said in a statement. “This is a fluid situation and details are still emerging. We are outraged by this. It is utterly unacceptable and we condemn it.

The Iranian government had a duty under international law to protect diplomats and London expected it to bring the situation under control and ensure the safety of the staff, the Foreign Office said.

Demonstrators waved flags symbolising martyrdom and held up portraits of Iran's Supreme Leader Ayatollah Ali Khamenei. An Iranian official told Reuters that the storming of two compounds in Tehran was not planned by the government.

“It was not an organised measure. The establishment had no role in it. It was not planned,” said the official, who declined to be identified.A separate group of protesters broke into a second British Embassy compound in the north of the city, the IRNA state news agency said, and seized classified documents.

Police tried to eject some 100 protesters from the north Tehran compound, what was once the embassy's summer quarters and which is now used to house diplomatic staff.

Riot police later moved in and mounted the main embassy gates, helping protesters climb back on the street outside, television pictures showed, and began to slowly clear demonstrators.

The incident followed Britain's imposition of new sanctions on the Islamic state last week over its nuclear programme.

London banned all British financial institutions from doing business with their Iranian counterparts, including the Central Bank of Iran, as part of a new wave of sanctions by Western countries.

In London, Foreign Secretary William Hague said Britain expected other countries to follow its lead in imposing financial sanctions on Iran and will take “robust” action if Tehran reduces their diplomatic relations.

Hague was speaking in a parliamentary debate just as news broke of the incident in Tehran and he made no comment on it. The Tehran government has two weeks to respond to the parliament's demand that it expel Britain's ambassador.

Trita Parsi, a U.S.-based expert on Iranian relations with the West said the attacks were very worrisome development.

“It is yet another sign that rather than permitting diplomacy to de-escalate the situation, tensions between Iran and the West are on the rise with Iran's continued nuclear work, sabotage, assassinations, sanctions and now the attack on the embassy,” he told Reuters.