Five men and a woman have been arrested on suspicion of the commission, preparation or instigation of acts of terrorism on the eve of 7/7.
The six arrests in London were part of a pre-planned intelligence-led operation by Scotland Yard's counter terrorism command along with armed officers, but were not linked to the Olympics, police said.
Three of the men, aged 18, 24, and 26, were arrested in east London. A force spokesperson said a 24-year-old man was tasered during his arrest, but did not require hospital treatment.
In west London a 29-year-old man was arrested in the street, while a 21-year-old man and the woman were arrested at a separate residential premises in area.
The six were taken to a south-east London police station
"Searches under the Terrorism Act 2000 are being carried out at eight residential premises in east, west and north London and one business premises in east London," the spokesman said.
"Public safety remains our overriding concern."
The arrests come after Jonathan Evans, the director-general of the Security Service, warned last month that Britain had experienced a "credible terrorist attack plot about once a year since 9/11".
"In back rooms and in cars and on the streets of this country there is no shortage of individuals talking about wanting to mount terrorist attacks here," he added.
"The threat is real and remains with us today."
The threat to the UK from international terrorism is currently rated substantial - the third highest of five levels.
The rating is set by the Joint Terrorism Analysis Centre (Jtac), based at MI5's headquarters at Thames House in central London, but is independent of the service.