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Tonight’s ride, though, was cast as part of the protest response to the Republican National Convention, and it ended in scores upon scores of arrests.

Citing safety hazards, Deputy Police Commissioner Paul J. Browne maintained that the busts on this ride weren’t political. “When they were happening before, they were obeying the law,” he said. “But in the last couple of months, this group has become increasingly aggressive.”

Browne seemed not to understand exactly what Critical Mass is. For the record, it’s a self-styled organic and leaderless phenomenon, a monthly ride of cyclists determined to reclaim a fair share of the streets from cars.

The rides have been hosted for the past seven years by Time's Up!, an environmental group. Browne got his players confused. “Before, it was these peaceful environmental groups doing it, but they seem to have been taken over by this other group—Critical Mass—that’s basically trying to take over the city,” Browne said, noting that last month thousands of bikers took over lanes on the West Side Highway and FDR Drive.

“That’s bullshit,” countered Times Up! coordinator Bill DiPaola. He argued that Critical Mass has been the same kind of ride month after month, lately growing by about 500 people each time. “I think the police are trying to scare people from demonstrating.”

Between 5,000 and 10,000 bikers participated in tonight's swing through midtown—despite the police having distributed flyers at the start of the ride in Union Square warning that anyone breaking traffic laws could be subject to arrest.

The ride ended outside St. Marks Church, in the East Village, and police in riot gear waded into the packed street, toppling to the ground one young biker who wailed in pain as they bent his arms behind his back for handcuffing. The crowd surged and a moving scrum broke out between police and demonstrators seeking to free the man. Police scooters plowed through the crowd, knocking into more bikers, prompting violent scuffles as a beat cop swung his baton menacingly, waving the crowds back.

Friday night bar crawlers joined the scene on Second Avenue, grooving to a mobile sound system as a police helicopter waved its high-powered beam overhead.

Police officials early this morning said at least 264 people were arrested, either during the ride—when cops strung plastic netting across the streets to corral riders and split up the mass—or on Second Avenue.