One of the Britons among about 500 people being held hostage by armed Chechens in a Moscow theatre has been freed.

About 40 rebels seized the theatre on Wednesday, threatening to shoot the audience and blow up the building if Russian security forces intervened.

But the Foreign Office has confirmed one Briton was among a number of hostages released.

About 200 people have so far been let go, Russian police say.

The Britons - part of one family - managed to telephone people outside when the attackers seized control of the auditorium, it has emerged.

A Foreign Office spokesman said: "I can confirm that one of the three Britons we know to be in the theatre has now been released and we are trying to get access to him."

Earlier British Prime Minister Tony Blair "completely and utterly" condemned the rebels.

Shooting warning

The group's leader, Movsar Barayev, said they are a Chechen "suicide" unit demanding the withdrawal of Russian troops from Chechnya.

The rebels warned, via a hostage, that they would start shooting captives if Russia failed to take their demands seriously.

Russian TV has shown live footage of a covered body being removed from the Moscow theatre.

Local news agencies have reported a woman had been shot dead by the gunmen but other reports indicated that a member of the audience may have died of personal medical problems.

The attackers reportedly released children and Muslims immediately

In a statement released by Downing Street, Mr Blair voiced support for the Russian government while saying he hoped to see a peaceful resolution to the siege.

"I completely and utterly condemn this act of terror which has affected hundreds of innocent people who were simply enjoying an evening's entertainment," he said.

"My thoughts are with all of them, including the British citizens we know to be inside the theatre."

Embassy support

Foreign Secretary Jack Straw said it was thought the Britons were part of one family and that British embassy staff had gone to the theatre to lend their support.

"It is a desperate situation," he said.

Negotiations have been taking place in the theatre foyer between rebels and a group consisting of Red Cross officials, members of the Russian parliament and a British journalist.

Earlier Grigory Karasin, Russian ambassador to Britain, told BBC Radio 4's Today programme that about 20 to 30 foreigners including Britons, Germans, Austrians, and Dutch, were being held.

The hostage-taking was the latest in a chain of terror events which began with the nightclub explosion in Bali and the Washington sniper - and could be linked to al-Qaeda, he told the Today programme.

"We have to understand that terror doesn't have any frontiers and appeasement.

"Playing political games with terrorists usually brings tragedy a bit later".

In a television statement Russian President Vladimir Putin said the rebels planned the attack in "foreign terrorist centres".

He blamed "the same criminals who have been terrorising Chechnya for the last few years" and said the authorities' main aim was to secure the release of the hostages.