The men, who were detained in a series of dawn raids across the country, are all understood to be Iraqi nationals considered by security sources to pose a direct threat to the UK.
They were arrested in houses in Croydon, south London, Derby and Wolverhampton after MI5 and officers from the Metropolitan Police anti-terrorism branch had held them under surveillance for more than a month.
The raids are being described as "significant" by security sources, but the Met said the investigation is not linked to either the London bombings on 7 July, or the failed attacks on the Underground a fortnight later.
A spokeswoman from Scotland Yard said that the men were being held on suspicion of the commission, preparation or instigation of acts of terrorism, under the Terrorism Act 2000 and were being questioned by detectives as part of an "intelligence-led investigation".
Reports that they were part of a cell affiliated with the Jordanian Abu Musab al-Zarqawi which was planning a series of car bomb attacks in the UK were dismissed by security and counter-terrorism sources.
Although no evidence of a specific plot has been uncovered, the arrests are understood to follow an intelligence assessment that the men posed a threat to British citizens, either in the UK or abroad.
The raids are thought to be part of a wider strategy aimed at disrupting the activities of suspected terrorists before they would have time to fully formulate plans.
Officers were carrying out detailed searches yesterday of the three houses where the men were arrested, but no discoveries have been made so far. The Scotland Yard spokeswoman said the searches were likely to continue for some time.
Three of the men were arrested at each of the houses in south London and Derby, while the other four were caught in Wolverhampton.
Neighbours of the house in Wolverhampton described the occupants of the raided house as quiet, but said that an unusual number of cars and vans would often be parked outside, blocking the street.
One neighbour, who did not want to be named, 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."
Officers were continuing to question the men in police stations in central London, the West Midlands and Derbyshire last night.
Under the Terrorism Act, they can be questioned for a maximum of 14 days before police must either charge them or release them.Reuse content