Police are searching three addresses in Blackburn and Accrington
Three men arrested on Thursday on suspicion of terror charges are being questioned by police.
Two were arrested at Manchester Airport, and the third was detained in Accrington. All three are in their 20s.
Police sources confirmed the arrests were not linked to previous terror attacks, said the BBC's Nick Ravenscroft.
The arrests were not at the "more serious" end of the scale for such offences, added our correspondent.
Detectives are searching the homes of the men, aged 21, 22 and 23.
Two of the addresses being searched are on Percival Street, Whalley Range, and Cromwell Street, Audley Range.
The arrests were a joint operation between Lancashire Police and Greater Manchester Counter Terrorism Unit.
Ch Supt Andy Rhodes, of Lancashire Police, said: "These arrests and subsequent searches of the nearby premises will be conducted with sensitivity and carried out as quickly as possible to ensure minimum impact on the three areas concerned.
"However, these types of inquiries can be complex and may take time to resolve.
"I would ask local residents to be patient with us and to be assured that we will keep them updated in relation to the investigation as and when we can."
Martin Clough, 32, a school caretaker from Cromwell Street, said: "It's quite disturbing. It's a small street and everyone gets on with everybody."
A spokesman for Jack Straw, Labour MP for Blackburn, said it was a matter for the police but Mr Straw was being kept informed.