Nine people have been arrested and four dogs have been seized in a series of raids in Lancashire as part of an investigation into dog fighting.
Seven addresses were targeted in the raids
More than 40 police officers visited seven addresses in Accrington.
A large industrial freezer with blood stains inside it was found at one of the properties, which police suspect may have been used as a fighting venue.
The seized dogs are now being held at RSPCA kennels and will be examined by vets for scars and other injuries.
Lancashire Police have urged anyone who suspects that dogs may be being bred to fight to contact them.
The properties raided on Thursday just after 0730 BST were on Willows Lane, Blackburn Road, Crossland Street, Richmond Road, Steiner Street and two addresses on Monk Street.
Officers launched the operation in response to information from the public about possible dog fighting in the area.
Det Insp Pete Simm said: "The operation demonstrates the commitment of Lancashire Constabulary, working closely alongside the RSPCA, to take firm action against anyone believed to be housing Dangerous Dogs or breeding them to fight."
The raids were the latest in a number of operations launched across the UK, particularly in Merseyside, since five-year-old Ellie Lawrenson was mauled to death in St Helens on New Year's Day.
"These are very, very serious offences that we are dealing with," Mr Simm said.
"Having said that, in light of what happened in Merseyside with the tragic consequences with the little girl there [Ellie] I think the public are a lot more aware now."
Four kinds of dog - pit bull terriers, Japanese Tosas, the Dogo Argentinos, and the Fila Brasileiros - are banned under the Dangerous Dogs Act 1991.
Cross-breeds of those dogs are covered by the law. RSPCA vets will also determine if any of the seized dogs are banned breeds.
Ch Insp Mike Butcher, from the RSPCA Special Operations Unit, said: "Dog fighting is an abhorrent, cruel activity that was banned in this country more than 150 years ago, but still continues today.
"It is an extremely barbaric sport... The dogs do literally kill one another and they rip one another apart - it couldn't be more barbaric.
"We are extremely pleased with the operation today and would like to thank the Lancashire Constabulary for their help and co-operation throughout."