NEW YORK -- The family, driving up the West Side on a sunny afternoon, suddenly found themselves surrounded by motorcycles. In front of their sport utility vehicle, behind it and buzzing past on either side.
A swarm of high-powered bikes had all but commandeered the three lanes of the northbound Henry Hudson Parkway on Sunday, and caught between them was the 33-year-old driver of a Range Rover, his wife and their 2-year-old daughter.
When one of the motorcycles apparently slowed to a near-stop in the center lane, it was struck by the vehicle, and its rider was knocked to the ground.
The vehicle and its occupants stayed put at first. But moments later, surrounded by the fallen biker's fellow riders, it barreled through the crowd, setting off a chase that would end in violence after the SUV became stuck in traffic on a side street in Washington Heights, police said. Several of the pursuing cyclists pulled the driver from the vehicle and beat him, police said.
As detectives were working Monday to piece together the events that led to the assault, a 6 1/2-minute recording captured on the helmet camera of one of the riders and posted to video-sharing websites showed the harrowing chase and the start of the attack.
Police Commissioner Raymond W. Kelly said the cyclists had been part of a planned, but unauthorized, event in which hundreds of riders gathered outside Manhattan and intended to descend en masse into Times Square. Under the name "Hollywood Stuntz," the loosely organized ride snarled Midtown traffic last year, Mr. Kelly said, "with well over a thousand motorcycles, dirt bikes, quads, four-wheel vehicles."
Jeff Forde, 24, a motorcyclist from Crown Heights, Brooklyn, said the gathering Sunday was meant 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, 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 massive informal ride, sending splinter groups tearing off in different directions.
For those who regularly drive in and around New York, such columns of motorcycle riders are a common and sometimes-frustrating feature of summer weekends. "We encountered a group of two to three hundred motorcycles," said Jon Phillips, recalling a drive on the Saw Mill parkway the previous Sunday. "It was rather unnerving."
The New York State Police arrested eight people in July after a "street stunt ride" of nearly 100 motorcycles stretched across two upstate counties. Police said many were seen driving into oncoming traffic, forcing other vehicles off the road and riding on sidewalks. That ride, too, was recorded and posted to the web.
Sunday's encounter between the SUV driver and the bikers began on 12th Avenue about 2 p.m., police said, as the motorcyclists filled the highway's northbound lanes in Upper Manhattan. Mr. Forde said that after the official ride was canceled at the last minute Sunday, he joined a large group that headed across the Brooklyn Bridge and up the West Side. There, they encountered the sport utility vehicle. He said the driver sped up when the riders approached and hurled obscenities at them.
As the video begins, the Range Rover can be seen in the center lane, surrounded on either side by motorcycles, some without license plates. One of the riders appears to slow down in front of the car and is struck, causing the group to stop and block the entire highway. Police said the collision appeared to have been inadvertent on the driver's part; the struck motorcyclist suffered broken legs, police said.
Seconds later, some cyclists, most wearing face-concealing helmets, crowded around the SUV. Police said some began attacking it, though that is not readily apparent in the video.
At least one rider present, Rene Towles, 43, offered a different account. "No biker became aggressive with the driver after the incident," said Mr. Towles, who belongs to a Brooklyn motorcycle club. "People were just trying to find out what just happened."
Moments later, the vehicle accelerates toward the riders who are blocking its path, crashing into several and appearing to roll over at least one motorcycle. Police said no other serious injuries were reported. For the next several minutes, the vehicle can be seen fleeing in the video, with cyclists in pursuit. After being forced to stop near the George Washington Bridge entrance, the driver is approached by one cyclist, who tries to open the driver's side door; the SUV bolts, knocking the man down.
The pursuers finally catch the vehicle in stopped traffic on 178th Street in Washington Heights, where, police said, the man was pulled from his car and punched by several assailants. The man, identified as Alexian Lien of Lower Manhattan, was treated for facial lacerations and bruising at New York-Presbyterian Hospital and released, police said; he was not charged.
Mr. Lien and his wife did not respond to messages seeking comment.
The video stops just as the assault begins, with a motorcyclist smashing his helmet into the driver's side window.