Police have thwarted several terrorist attacks in the UK during the past two years, according to Scotland Yard.
Anti-terrorism efforts were stepped up after 11 September 2001
Officers believe a missile strike on a plane at Heathrow was averted when 450 troops were deployed in February.
Other unspecified attacks are also thought to have been stopped since security was stepped up after the 11 September terrorist attacks.
Last week London Mayor Ken Livingstone said four attempts to "cause mayhem and take life" in the city had been foiled.
Metropolitan Police Assistant Commissioner David Veness told BBC News efforts by anti-terrorist officers had stopped "dire events" including some within the UK.
He said: "We need to confront, and it's an unpleasant reality, that British experience of terrorism, which was at the level of the car bomb, now needs to embrace activities that are regretfully unequivocally aimed at doing greater public harm. Even, sadly, murder on a mass scale."
Asked whether a UK attack could be imminent, he said: "The fact attacks have occurred within the European theatre aimed at Western interests within the last two to three weeks indicates the nature of the challenge."
Scotland Yard also believes hundreds of potential al-Qaeda sympathisers are in Britain.
Police now consider it a question of "when, not if" a successful terror attack will be launched on the UK, Mr Veness told BBC News.
Scotland Yard sees three linked tiers of terrorist threat - large-scale al-Qaeda attacks, associated groups in north Africa, Indonesia, the Philippines and across south-east Asia and individuals such as "shoe bomber" Richard Reid.
About 500 people in the UK have been arrested under anti-terror laws.
Amnesty International UK warned detaining suspects indefinitely without charge was creating a "Guantanamo Bay in our own back yard".
But police defended their record saying they do not arrest people under the Terrorism Act unless there is specific intelligence against them.
Of the 500 people arrested, 90 have been charged with terrorism offences, police say.
Around 100 have been charged with other crimes, while 50 have been handed over to the immigration services.
The level of co-operation between police, the security services and foreign agencies, is unparalleled, according to Scotland Yard.