Protesters stormed Britain's embassy and another diplomatic compound in Tehran on Tuesday, sparking international alarm and dramatically raising tensions with the West over Iran's nuclear programme.

Young men chanting "Death to Britain" entered both properties, trashing offices, stealing documents and violently defying police efforts to remove them for several hours, according to an AFP journalist and Iranian media.

Six diplomats sequestered for more than two hours inside a building in Britain's diplomatic compound in the north of the capital by hundreds of protesters were finally freed when diplomatic police intervened, the Fars news agency reported.

Inside the embassy in the city centre, several protesters scattered documents and set them alight, witnesses told AFP. One protester was seen looting a portrait of Britain's queen.

The incursions, which took place during protests planned days in advance, only came to an end late Tuesday when special police units were deployed.

Britain said it took the storming of its embassy "extremely seriously" and warned of "further and serious consequences" for Iran.

"We hold the Iranian government responsible for its failure to take adequate measures to protect our embassy, as it is required to do," British Foreign Minister William Hague said in a statement after a meeting of the government's crisis committee chaired by Prime Minister David Cameron.

Hague said that although the Iranian foreign minister had apologised for the incidents, "this remains a very serious failure by the Iranian government."

The United States, which cut off diplomatic ties to Iran after students stormed its own embassy in 1979, taking 52 Americans hostage for 444 days, said it condemned Iran "in the strongest terms."

President Barack Obama said the storming of the embassy was "not acceptable" and that "all of us are deeply disturbed" by the event.

"They have a responsibility to protect diplomatic outposts," Obama said, referring to the Vienna Convention, an international law regarding the neutrality and protection of missions.

For rioters "to basically be able to overrun the embassy and set it on fire is an indication that the Iranian government is not taking its international obligations seriously," he said.

A UN Security Council statement also highlighted the Vienna conventions.

The council "called on the Iranian authorities to protect diplomatic and consular property and personnel, and to respect fully their international obligations in this regard."

Russia -- Iran's closest ally -- called the embassy attack "unacceptable and deserving condemnation."

Iran responded quickly to the international outcry, with the foreign ministry issuing a statement expressing "regret for certain unacceptable behaviour by a small number of protesters."

It emphasised what it said was the Iranian government's commitment "to protecting the rights of diplomats and diplomatic locations."

Tehran police chief General Hossein Sajadinia said several protesters had been arrested and would face the courts.

Tuesday's demonstration had been organised days earlier, when Iranian lawmakers passed a law to kick out Britain's ambassador in retaliation for London's announcement last week that it was cutting off all relations with Iran's financial sector.

The measure was part of a raft of sanctions coordinated with the United States and Canada to pressure Iran over its nuclear programme, which Western nations fear is being used to build an atomic arsenal.

Britain has warned it will act "robustly" if its ambassador is expelled. Its embassy on Monday had called on Iranian authorities "to ensure that British embassy staff and premises in Iran are adequately protected" during Tuesday's protest.

The demonstration started with hundreds of protesters gathering in the street outside the embassy.

A smaller group clambered up the wall and got inside, ripping down Britain's flag and replacing it with Iran's, and breaking windows and trashing offices and vehicles.

After initial inaction, police went in repeatedly to chase out the protesters, finally succeeding after firing tear gas, according to the Fars news agency.

Several protesters were injured, one critically. Several officers were also hurt.

Between 100 and 300 protesters got into Britain's other compound in the north of Tehran, blocking the six British diplomats and taking what they said were "espionage" documents that they set fire to.

The looting of the British embassy and compound came ahead of an EU foreign ministers' meeting on Thursday that was expected to reveal new sanctions against Tehran.

The European Union and the United States said on Monday they were considering extra measures to pressure Iran on its nuclear activities.

The West's suspicions that Iran was seeking an atomic bomb were crystallised in a report by the UN nuclear watchdog this month that strongly suggested Tehran had researched nuclear warheads.

Iran has repeatedly said its nuclear programme has only peaceful, civilian purposes, and has warned it would respond to any military attack by raining missiles on Israel and Turkey.