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

Protesters remained in both properties, trashing offices, stealing documents and defying police efforts to remove them, according a journalist and Iranian media at the scene.

Six British diplomats sequestered for more than two hours inside a building in Britain's diplomatic compound in the north of the capital were finally able to get through a crowd of hundreds of protesters after intervention from diplomatic police, the Fars news agency reported.

Inside the embassy in the city centre, several protesters scattered documents and set fire to them, witnesses informed.

The demonstrators were showing their anger at Britain's announcement last week that it was cutting off all relations with Iran's financial sector as part of a raft of sanctions coordinated with the United States and Canada.

Iran responded by passing a law yesterday to expel Britain's ambassador within the next two weeks. Britain has warned it will act "robustly" if Iran's foreign ministry complies.

Police, who had already chased out a group of more than 20 protesters from the embassy grounds, fired tear gas to try to dislodge the group inside the mission, Fars reported.

The news agency said several protesters were injured, one critically. Several officers were also hurt.

Protesters had ripped down Britain's flag at the embassy, replacing it with Iran's, and broken windows and vandalized buildings and vehicles.