Ten people were arrested Monday as police in central and northern England carried out a series of raids under Britain's main anti-terrorist law, Greater Manchester Police said.
"Ten people in total have been arrested on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism," a Greater Manchester Police spokesman told AFP.
"We appreciate the public interest in this but are unable to provide more details at this stage," he said.
Police declined to comment on a report on Sky News television that the raids were linked to an apparent terrorist threat against the Trafford Centre, a large shopping and entertainment complex in Manchester.
On March 30 a similar police swoop in and around London saw nine people arrested under the Terrorism Act 2000 and a large quantity of ammonium nitrate fertiliser - a possible bomb-making ingredient - seized.
Those raids were the biggest in Britain since the January 2003 discovery of traces of the deadly poison ricin in a north London apartment.
Four police forces - Greater Manchester, West Midlands, South Yorkshire and Staffordshire - participated in Monday's operation, the Manchester police spokesman told AFP by telephone.
Witnesses said one of the raids targeted a flat, occupied by a single man for about a year, situated above a takeaway restaurant owned by Iraqi Kurds on Upper Brook Street near Manchester's city centre.