Two further men have been arrested in the early hours of Thursday in a vehicle on the M6 motorway.
The men are aged 33 and 42. The former was arrested on suspicion of being a member of a proscribed organisation, supporting a proscribed organisation and encouraging terrorism.
The latter was arrested on suspicion of assisting an offender. Both men remain in custody in a central London police station.
Updated — Sept. 25, 9:40 a.m. BST
The Metropolitan police have arrested nine men as part of an ongoing investigation into Islamist-related terrorism.
According to The Times, the men are linked to a group formerly known as al-Muhajiroun.
The men, aged between 22 and 51, have been taken to police stations in central London.
The Press Association is reporting that one of the men being held is radical preacher Anjem Choudary.
The police said the arrests and searches were part of an "ongoing investigation into Islamist-related terrorism and are not in response to any immediate public safety risk".