London - Police and emergency services took part in the biggest counter-terrorism drill held in London to date on Tuesday, with about 1,000 officers testing their response to a potential militant attack on the British capital.
Britain is already on a state of high alert following an attack by a gumman at Tunisian holiday resort last week which killed 39 people, most of them British tourists.
The two-day exercise was planned before that, following attacks in Paris in January in which Islamist gunmen killed 17 people in a rampage, a goverment spokesman said.
But following the Tunisia violence, the government has warned the public that attempts to stage an attack in Britain were possible.
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Most of the drill will take place out of public view in several locations across London. But reporters saw about half a dozen armed police officers, some clutching shields, entering a disused underground station in central London to hunt down "terrorists" as gun shots rang out on Tuesday morning.
Other personnel could be seen treating injured "victims" on a pavement.
Deputy Assistant Commissioner of London's Metropolitan Police, Maxine de Brunner, told Reuters the scale of exercise, was larger than anything undertaken previously in the capital.
It involved London's transport authority, local government and the Ministry of Defence and will see the government's emergency Cobra committee convened.
Prime Minister David Cameron told parliament on Monday the training exercise would "test and refine the UK's preparedness for dealing with a serious terrorist attack".
It also takes place a week before the 10th anniversary of the suicide bombings on London's transport network by four British Islamists which killed 52 people.