Police are continuing to question nine men arrested in a series of anti-terrorism raids.
Three residential addresses are being searched
Three men were held in Croydon, London, four in Wolverhampton and three in Derby - one was subsequently released.
The men were held on suspicion of the commission, preparation or instigation of acts of terrorism. The raided properties are being searched.
The arrests were not connected to the investigation into the July London bombings, Scotland Yard said.
Officers raided three residential addresses following an "intelligence-led investigation involving police and the security service", said the Metropolitan Police.
The force's anti-terrorist branch was helped by local officers from the Met, West Midlands and Derbyshire police forces.
The arrested men were taken to police stations in the West Midlands, Derbyshire and central London.
Forensic searches of the addresses are expected to continue for some time.
A resident in Lewis Avenue, in the Moseley Village area of Wolverhampton, said up to 20 people had stayed at the address raided there at any one time.
One man said: "There was always activity in the early hours of the morning. Men would be carrying plastic bags and packages to and from the house."
A woman added: "They were quiet, kept themselves to themselves, but I always thought there was something strange going on."