The men are being held under controversial anti-terror laws
Eleven people have been arrested on suspicion of terrorism, police have said.
Six men believed to be Algerian were held in London and four men, also Algerian, were held in Manchester following dawn raids, according to Scotland Yard.
Another 25-year-old man was arrested on Tuesday afternoon in London.
The men were arrested in co-ordinated raids.
Police have also begun searching two business premises in London.
A police spokesman said: "This operation is part of our extensive and on-going inquiries into terrorism."
At least five of the detained men are aged in their 30s.
Searches under way
The seven arrested in London are being held at a central London police station under Section 41 of the Terrorism Act 2000.
They are being held on suspicion of being involved in the "commission, preparation or instigation of acts of terrorism".
They were arrested at several locations by officers from the Metropolitan Police's anti-terrorist branch.
At about 0530 BST a 32-year-old man was arrested at a house in Manchester by officers from Greater Manchester Police.
The man is en route to London where he will be questioned by officers from the anti-terrorist branch, said Scotland Yard.
Three others were also arrested in Manchester in the same raid.
Police are searching the addresses they raided.
Security sources said the arrests were based on intelligence received and were the result of joint work between the police and the MI5, according to BBC home affairs correspondent Danny Shaw.
They were part of ongoing enquiries into international terrorism and not in response to a specific threat.