A teenager who allegedly beat a police officer unconscious during a protest outside Madison Square Garden was arrested at another demonstration in Union Square Tuesday. Jamal Holiday, 19, was identified using a videotape of Monday night’s incident. As protestors clashed with police, Detective William Sample was pulled off his scooter and was repeatedly punched and kicked. Sample has since been released from the hospital and is expected to make a full recovery. Holiday is charged with second-degree assault on a police officer.