Seven of the arrests were in the Manchester area and one was in Merseyside. Another raid reportedly took place in Birmingham, according to the BBC.
The detainees are reportedly suspected of facilitating terrorism overseas.
"Officers have arrested eight people and searches are now taking place. About 500 officers have been involved in the action throughout the UK," said police in a statement.
The swoop began before dawn on Wednesday and is being led by the Greater Manchester Police anti-terrorist unit.
The raids come after a months-long investigation, according to reports, and properties have been searched using warrants issued under the Terrorism Act.
A spokeswoman for the Greater Manchester force said an "extensive operating targeting individuals suspected of facilitating terrorism abroad" was being conducted, however gave no further details on the exact nature of the alleged offences.
A Home Office spokeswoman said the arrests were part of an operation by the Immigration Service and police.