Police say they're investigating after witnesses reported that a rough encounter between a skateboarder and a San Francisco police officer who was breaking up a competition may not have been accidental.

A video clip shows a skateboarder zipping downhill before he hits the officer, strikes a stationary patrol car and lands hard on the ground as the crowd cries in outrage.

Witnesses tell the San Francisco Chronicle that the officer appeared to put his left arm in the skateboarder's path.

The newspaper reported that hundreds of participants and spectators squared off against officers Tuesday.

Officer Robert Rueca says police are working to identify the injured skateboarder. They checked with hospitals but were unable to find him.

The officer in the video was taken to a hospital for unspecified injuries.