Multiple blazes started near Broadhead Road, Darwen shortly before 16:00 BST on Sunday.
Lancashire Fire and Rescue Service incident commander Jerry Cragg said "numerous individuals" were "spotted at the time remaining in a fire setting".
Lancashire Police has launched an investigation.
The incident commander said they were aided by crews from Greater Manchester and mountain rescue teams.
More than 60 firefighters have been tackling "intense" fires on moorland in Lancashire started deliberately, said the fire service.