As detectives were working on Monday to piece together the events that led to the assault, a six-and-a-half-minute recording captured on the helmet camera of one of the riders and posted to video-sharing Web sites showed the harrowing chase and the beginning of the attack.

The police have made no arrests in the case. The police commissioner, Raymond W. Kelly, said the motorcyclists had been part of a planned, but unauthorized, event in which hundreds of riders gathered outside of Manhattan and intended to descend en masse into Times Square. Under the name “Hollywood Stuntz,” the loosely organized ride succeeded in snarling Midtown traffic last year, Mr. Kelly said, “with well over a thousand motorcycles, dirt bikes, quads, four-wheel vehicles.”

Image CONFRONTATION The S.U.V. stayed put at first, the video shows. But moments later, surrounded by the fallen biker’s fellow riders, it shot through the crowd, plowing through motorcyclists and setting off a frantic chase up Manhattan’s West Side.

Jeff Forde, 24, a motorcyclist from Crown Heights, Brooklyn, said the gathering on Sunday was intended to celebrate the end of the summer rides and to mark a moment of fraternity among rivals. “Everybody just comes out for that one last ride,” Mr. Forde said. “You got all these different crews, all these groups, who most of the year wouldn’t be seen next to each other. It’s one big family thing.”

This year, though, the police were ready, with checkpoints set up to inspect the bikes and their riders at bridges and tunnels into Manhattan. At least 15 people were arrested, mostly on vehicular charges, Mr. Kelly said, and 55 motorcycles were confiscated. This was enough to break up the colossal informal ride, sending splinter groups tearing off in different directions.