A group of suspected Islamist terrorists are being quizzed by detectives amid fears the men planned to launch an attack on British soil.
Armed police fired special ammunition designed to breach doors or burst tyres at a car during one of a series of dramatic anti-terror raids across London.
Counter terrorism officers forcibly stopped the vehicle on Mansell Street, near the Tower of London, with so-called Hatton rounds and arrested two men, both aged 25.
A 28 year old man was also arrested in Westbourne Grove and a 29 year old was held in Peckham Street, south-east London, during last night's MI5-assisted operation.
All the men have been arrested on the suspicion of the commission, preparation or instigation of acts of terrorism, police say.
More top news
Pictures of today's events unfolding following a major security incident at the Houses of Parliament.
At least two people have died after a knifeman brought terror to the heart of Westminster.
The Cobra committee brings together government ministers with senior officials of the emergency services and security agencies.