Homes across Preston were raided as part of a major investigation into the supply of heroin and cocaine.
A total of 18 men were arrested in the co-ordinated raids which also took part in Cumbria and Merseyside at the same time.
Police targeted addresses in Moor Hall Street and Broughton Street in Plungington; Dove Street and Castleton Road in Deepdale; Nevett Street, Callon; Bowland Road, Ribbleton; Moss Lane in Hesketh Bank, near Preston; and three streets in Blackburn.
Seven men were arrested in Preston including three 30-year-olds, two aged 27, a 31-year-old and a 26-year-old.
The operation, codenamed Operation Oak, began at around 7am yesterday and involved officers from Lancashire’s Serious and Organised Crime Unit, the Targeted Crime Units from Preston and Blackburn and colleagues from Cumbria and Merseyside.
The men, all aged between 26 and 33, were arrested on suspicion of drug supply offences.
They were all taken to police stations across Lancashire for questioning.
Det Chief Insp Tim Leeson, who is heading the operation, said: “The raids were part of an ongoing investigation into the activities of a suspected organised crime group and a drugs supply chain that has seen heroin and cocaine being moved across, and sold in, Lancashire, Cumbria and Merseyside.
“While crime as a whole is low in Lancashire, the activities of serious and organised criminals can have a dramatic effect on our communities.
“It can ruin lives, fuel other crime, such as burglary and robbery, and can install fear in our neighbourhoods.
“Lancashire Constabulary is committed to tackling organised crime and we would ask that communities help us in doing this.
“It is incredibly important that people continue to provide the police with information so that we can keep them safe and look for ways to prevent organised crime groups from operating in the future.”
A spokesman for Cumbria Police said four were arrested in Barrow.
Anyone with information should contact Lancashire Police on 0845 1253545 or Crimestoppers on 0800 555 111.