Two men, aged 30 and 34, and a 29-year-old woman were arrested at about 07:00 BST at addresses in West Yorkshire while a third man, 22, was arrested at an address in the West Midlands.
"The four people were arrested on suspicion of the commission, preparation, or instigation of acts of terrorism under the Terrorism Act 2000," a Met spokesperson said.
"They are being taken to a central London police station where they will be kept in custody and will be interviewed by officers from the Met's counter terrorism command."
Media reports citing unnamed sources suggest that the detained woman is the widow of the presumed ringleader of the July 7th attacks, Mohammad Sidique Khan.
Sky News and the BBC report that Hasina Patel, 29, from Dewsbury, was among those arrested in West Yorkshire.
Chief Superintendent Barry South of West Yorkshire police said he "can't speculate on who has been arrested and who hasn't" but insisted as much care as possible was being taken to be sensitive to the local community.
"The issue for me is to make sure the investigation is done with the minimum impact on the local community," he said, adding that the police have acted on feedback from local residents in the way they conducted today's operation.
Police are also conducting searches at five West Yorkshire addresses – two houses in Dewsbury, two houses in Beeston and one house in Batley – and at two flats in Birmingham.
Scotland Yard says no armed police officers were used in the "pre-planned, intelligence-led operation".
Three other men have previously been charged in relation to the attacks, which occurred on three London Underground trains and a local bus.
Mohammed Shakil, 30; Sadeer Saleem, 26; and Waheed Ali, 23, of Beeston, Leeds, have been charged with conspiracy to cause explosions and have been remanded in custody until June 8th.
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