About 1,000 bicyclists jammed the streets of Hollywood overnight, causing disruptions to traffic, police said.
The bicyclists rode through red lights and "stampeded like a herd of wild buffalo," said the watch commander at the Los Angeles Police Department's Hollywood Station.
No arrests were made. Though no injuries were reported, he said the late-night riders were dangerous and someone would eventually get hurt.
Similar problems were reported earlier around Third Street and La Brea Avenue.
Local news from across Southern California
The riders were believed to be part of a Critical Mass ride, which, according to Critical-Mass.info, is a "monthly bicycle ride to celebrate cycling and to assert cyclists' right to the road."
According to the site, Critical Mass has no leaders and no central organization, it is not an organization, but rather, an idea and an event.