Police said the disorder lasted an hour
A brawl involving up to 500 youths broke out in Blackpool resulting in three police officers being injured.
Police were pelted with missiles, including cans, as they responded to reports of disorder in Bank Hey Street and Victoria Street at about 21:00 BST on Friday.
Three officers received minor injuries with police eventually bringing the situation under control after an hour.
Twelve people, aged 13 to 22, have been arrested.
Extra patrols will take place over the rest of the weekend and next Friday.
Those arrested for a variety of public order offences are currently in custody, Lancashire Police said.
Det Sgt Mark Dickinson, of Blackpool Police, said: "We understand that this incident may have left people upset and frightened and we are determined to trace those responsible.
"This level of violence is completely unacceptable and will not be tolerated."