Police say they have disrupted a known terrorist group after making arrests in raids across England on 24 May.
Homes were searched in the raids
They are holding one man under immigration powers. Two others, held under the Terrorism Act after raids on 19 addresses, have been released.
Five other people are still being held under immigration legislation.
More than 500 officers took part in raids on addresses in Manchester, Liverpool, Middlesbrough, Bolton, Birmingham and London.
The operation involving five police forces was headed by Greater Manchester Police.
Chief Constable Michael Todd said: "Greater Manchester Police's Anti-Terrorist Unit has been carrying out a detailed investigation into facilitation of terrorist activity overseas.
"This is a major investigation that will continue for some considerable time.
"The activity last week was the latest phase of the investigation and as a result of the operation, a significant amount of material was seized, which is being reviewed by detectives.
"It is clear that the activity last week has disrupted a known terrorist group.
"Six of the eight arrested continue to be detained in custody pending deportation by the Immigration Service as they are a threat to national security."
The other five men are being held under the home secretary's powers to deport individuals whose presence in the UK is "not conducive to the public good for reasons of national security".
The eight detained men are reported to be Libyans, and the operation was aimed at those suspected of aiding attacks on targets in Iraq.