Hard-line Iranian students stormed the British diplomatic compounds in Tehran on Tuesday, bringing down the Union Jack flag and throwing documents from windows in scenes reminiscent of the anger against western powers after the 1979 Islamic Revolution.

The mob surged past riot police into the British Embassy compound which they pelted with petrol bombs and stones two days after Iran's Parliament approved a bill that reduces diplomatic relations with Britain following London's support of recently upgraded western sanctions on Tehran over its disputed nuclear programme.

Less than two hours later, police appeared to regain control of the site. But the official IRNA news agency said about 300 protesters entered the British Ambassador's residence in another part of the city and replaced British flags with Iranian ones.

The British Foreign Office denounced the melee and said Iran had a “clear duty” under international law to protect diplomats and offices.

“We are outraged by this,” said the statement. “It is utterly unacceptable and we condemn it.”

It said a “significant number” of protesters entered the compound and caused vandalism, but gave no other details on damage or whether diplomatic staff was inside the embassy, though the storming occurred after business hours.

The semi-official Mehr news agency said embassy staff had left the compound before the mobs entered, but it also said those who occupied the area had taken six staff as hostages. It did not give their nationalities and the report could not immediately be confirmed.

The protesters broke through after clashing with anti-riot police. “Death to England,” some shouted in the first significant assault of a foreign diplomatic area in Iran in years. More protesters poured into the compound as police tried to clear the site.

Smoke rose from some areas of the embassy grounds and the British flag was replaced with a banner in the name of 7th century Shia saint, Imam Hussein. Occupiers also tore down a picture of Queen Elizabeth II.

The occupiers called for the closure of the embassy calling it a “spy den” — the same phrase used after militants stormed the U.S. embassy in Tehran in 1979 and held 52 hostages for 444 days. In the early moments of the siege, protesters tossed out papers from the compound and hauled down the U.S. flag. Washington and Tehran have no diplomatic relations since then.

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