The fight kicked off in Norrköping, eastern Sweden, at around 10pm on Monday. One witness told the TT news agency that he saw someone distributing knives before the fighting began. Police rushed to the scene, where fighting continued despite the officers' presence.
Two middle-aged brothers were shot dead in the brawl. Many others were injured, with one still in hospital with serious gun shot wounds.
"It was a real mess. Many people were hit in one way or another. The police who were there said it was a extremely messy, some of them said it was the worst they had seen," Fredrik Kliman of the Östergötland police told the TT news agency.
Police suspect more people could be hurt, but that they escaped the scene rather than seeking medical care.
"The entire picture remains unclear," Kliman said, adding that knives and knuckledusters were also used in the brawl.
Five men have been held by police, suspected of either murder, abetting murder, or attempted murder.
Kliman added that he did not think the shooting was related to a similar gang-related incident in the area previously. Police have launched an investigation, and will be speaking with the witnesses in the area who may have information.