ROUBAIX, France — Three spectators were seriously injured Sunday when a motorcycle crashed into fans watching the Paris-Roubaix cycling race, a one-day classic won by Belgium's Tom Boonen.

Three people were taken by helicopter to hospitals in Lille and Valenciennes for further scans, two for abdominal injuries and a woman who was briefly knocked unconscious, said Philippe Sudres, a spokesman for race organizer Amaury Sport Organization.

"Doctors do not think their (lives) are in danger, but we have to wait for the results," Sudres said.

About a dozen others needed medical treatment, Sudres said.

The motorcycle belonged to race organizers to provide regular time checks for riders, he said. It appeared to lose control and swerved sideways into the side of the road.

Boonen broke away from the pack about 10 miles from the finish and comfortably won one of cycling's toughest classics a third time. Italy's Filippo Pozzato was 47 seconds behind, and Norway's Thor Hushovd was third, 1 minute, 17 seconds back.

Last year, Boonen beat former champion Fabian Cancellara of Switzerland in a sprint finish. He also won in 2005.

This year's race covered 161 miles and ended in the northern French town of Roubaix. The route featured a 33-mile ride over punishing cobblestones.

Hushovd and Leif Hoste of Belgium and Juan Antonio Flecha of Spain lost valuable time after falling at one of the several cobblestone sections. Hushovd had looked strong enough to challenge Boonen on the final stretch, but his chances of victory ended when he went too wide on a turn and his front wheel slipped, causing him to tumble off his bike.

Boonen pulled away and Pozzato was unable to match his burst of acceleration, quickly losing 10 seconds and then gradually more time. As he approached the finish in the Roubaix velodrome, Boonen looked several times over his shoulder but no riders were close.