The incident occurred Friday evening in Times Square at the Critical Mass ride, a monthly protest of urban reliance on motor vehicles.
The video clip, posted anonymously on the Internet on Sunday, shows the officer standing in the street as bikes whiz past. The officer moves toward the cyclist, identified in a criminal complaint as Christopher Long, and violently knocks him to the ground in front of crowds of people. Another officer comes over, and the two officers wrestle with Long before handcuffing him.
Craig Radhuber said he was riding about 3 feet behind Long when he was hit.
"It was unprovoked," Radhuber said. "It was like the cop was waiting for him and then just checked him as he got closer. I couldn't believe it."
The officer in the video was placed on desk duty pending the outcome of a police department investigation, chief police spokesman Paul Browne said.
The video, shot by a bystander, was posted on YouTube.