Police are continuing to question four men arrested following a major investigation into alleged international terrorism.
The four men are being held at Paddington Green police station
The arrests are linked to the arrest of a man at Manchester airport on 6 June.
One man, 28, was detained on Tuesday in south-east London and two men, aged 29 and 21, on Monday in east London.
The fourth man, aged 21, is from Bradford. Three of the men are being questioned in West Yorkshire and the other remains in London.
All four men are British citizens and were arrested on suspicion of the commission, preparation or instigation of acts of terrorism under the Terrorism Act 2000.
Margaret Gilmore, BBC home affairs correspondent, said the arrested men were being held at Paddington Green police station in west London.
She said: "Police are trying to establish whether they [the arrested men] were giving support to an alleged al-Qaeda cell in Canada. The operation leading to their arrests involved West Yorkshire Police, MI5 and Scotland Yard."
Police said the arrests were not linked to the Forest Gate raid in east London or the 7 July attacks last year.
Scotland Yard said the police operation did not involve firearms officers. The arrested men can be held for 14 days, after which they have to be charged or released.
The arrests are linked to the investigation, led by West Yorkshire Police, in which a man and a youth were charged earlier this month with conspiracy to murder and conspiracy to cause public nuisance by using poisons or explosives.
Aabid Hussain Khan, 21, of West Yorkshire, was arrested at Manchester Airport in early June and later charged under Section 57 of the Terrorism Act.
He appeared in court on 15 June and was remanded in custody by Bow Street magistrates until 30 June to appear at the Old Bailey.
Khan appeared alongside a 16-year-old boy, who cannot be named for legal reasons, who was charged with conspiracy to murder and conspiracy to cause a public nuisance by use of poisons and/or explosives.