Another man is also reported to have required treatment after the brawl on the open street outside a bar on Södermalm in central Stockholm.
According to witness reports some 15 members of the Red and White Crew, a supporter club of the Hells Angels, stormed the El Cocodrilo restaurant on Ringvägen.
Armed with knives, tear gas and firearms the group attacked a rival gang, reported by the police to be either Outlaws or Bandidos.
“I heard screaming and shouting, then I saw a mob run in and out from the restaurant. Then they came out carrying a half-naked guy, who fell to the ground and who they continued to assault,” an eye witness told the TT news agency.
The witness watched as the man was beaten bloody outside the restaurant before the police managed to break up the fight.
There are reports of shots being fired inside the restaurant, but these are denied by the police.
“They waved a weapon, but no one has been shot,” said Stefan Nordemark at Stockholm police.
The police detained a person during the night, with a second man subject to a warrant for his arrest.
“We have conducted a number of raids, including a club premises. We are working on the case and it feels as if we have a good situation.”