Members of the Hells Angels motorcycle gang stand accused of viciously beating at least eight people at a private party held by a club in southern Sweden earlier this year, the Swedish news agency TT reported.
Several witnesses called police for help saying gang members were beating people with iron rods.
"You need to hurry, bloody well hurry," one caller said on a recording played in court.
Another caller shouted into the phone: "The Hells Angels are knocking everyone out, we have children here and family, you really have to come quickly."
Prosecutor Asa Adolfsson said gang members wanted the biker club shut down but he didn't know the reason why.
The trial is expected to last as long as seven weeks.
It is being held in Gothenburg District Court under tight security.
Chipotle plans first price increase in 3 years
Putin thinks Obama would save him if he were drowning