Los Angeles officials vowed to arrest and prosecute cyclists for incidents like the one Friday night in which hundreds of riders swarmed a Ralphs parking lot and broke out beer and marijuana, with some riding into the store and through the aisles.

Participants would be cited for violating traffic rules that endanger motorists or pedestrians, said Cmdr. Andy Smith. Those who engaged in violence would be immediately arrested.

The warning during a news conference at the downtown LAPD headquarters comes ahead of a mass ride scheduled for Friday and is aimed at protecting the rights not only of cyclists but also of those who share the roads and sidewalks with them.

LAPD officials have been trying to forge better relations with bike activists, even advocating department-wide training to highlight the rights of bicyclists on the road and ensure that officers know how to deal with incidents involving bikes.

The improved communications follow an incident in Hollywood in May in which a video appeared to show an officer kicking at a passing bicyclist during a protest ride against BP over the Gulf of Mexico oil spill.

The video footage, which was posted on YouTube with the title "Hollywood Cops Attack Bike Riders," inflamed many bicycle activists, who complained about mistreatment at the hands of the LAPD.

Ramon Martinez of the Los Angeles County Bicycle Coalition said Wednesday that better communications have led to a reduction in tension with police as well as a better experience for cyclists.

"It's been increasingly positive in the last six months and is heading in the right direction," Martinez said.

Smith said Wednesday that it was important to remind participants in mass bike rides that they must obey the law and respect others' rights. Increasing numbers of cyclists appear to have been ignoring that warning during recent rides, he said.

Last Friday in Venice a group calling itself "crank mob" broke away from the main body of 600-800 riders and swarmed a Ralphs parking lot on Lincoln Boulevard.

During the impromptu party, deejays began playing music as some in the crowd began drinking beer and smoking marijuana, Smith said. About a half dozen broke off and rode into the store.

Three people were cited and a juvenile was arrested after allegedly hitting a police officer.

Smith said the warning was aimed at those who are failing to adhere to the spirit of the riders.

"We are here to ensure the safety of everyone," Smith said. "But there are those that don't listen to the organizers and they certainly don't listen to us. So our only aternative in this case is to stop them, cite them or make arrests."