The supects are believed to be at Paddington Green police station
A woman and three men were arrested on Friday after a series of raids across the country and taken to a police station in central London, understood to be the high security Paddington Green.
Scotland Yard have confirmed that a man aged 27 and a 25-year-old woman arrested in Berkshire, and a man in his 40s arrested in Birmingham, can now be held in custody until Wednesday.
Magistrates ordered the extension under the Terrorism Act 2000 and police would be allowed to apply for a further extension until Friday.
A 29-year-old man arrested in west London has been released without charge.
Police have refused to confirm the identities of any of the arrested people.
In Birmingham on Saturday a single police van was parked on Highgate Street, near the city's Central Mosque.
Residents said police had removed property from a house in Dymoke Street in the Highgate area.
Terrorist attacks
The arrests are the first in Britain, although it was reported that officers raided an address in Brixton, south London, last weekend at the request of the FBI. No arrests were made.
Metropolitan Police Commissioner Sir John Stevens said on Thursday that some of the estimated 19 hijackers visited Britain before heading to the US to carry out the terrorist attacks.
BBC correspondent Margaret Gilmore said: "The hijackers came through several months ago, but security sources said there could be others still here who helped with travel arrangements, with funding and with communications."
Other European countries are continuing their investigations into the hijackers responsible for last week's attacks, their possible associates and their sources of funding.
Belgian police arrested two men and seized a huge store of chemicals in their Brussels flat.
Charges
The police raid, which was made on Thursday night but only revealed on Saturday, was linked by the Belgian prosecutor's office to the earlier arrest of two suspected Islamic militants.
Seven people have been arrested in France. German police have arrested one man and issued photographs of two others wanted for questioning.
At least 50 people have been arrested by the FBI in the US in connection with the attacks.
On Friday evening federal prosecutors said they had charged a man with trying to fly into Chicago with an illegal passport and airline uniforms on the day of the hijacking attacks.
Thirty-three other people in the US have been charged with immigration violations. Department of Justice officials believe many of them may have information about the hijackings.