Sky 4 video showed markers indicating shell casings on the ground near the corner of Fort Duquesne Boulevard and Seventh Street.
Officers responded to a report of a car crash at about 3:45 p.m. and learned that a shooting had occurred after some sort of dispute between the driver and a man on a bicycle, police spokesman Chris Togneri said.
The car drove onto the sidewalk, where it hit a Port Authority bus shelter, and the driver got out and shot at the bicyclist, who was struck in the leg, Togneri said.
The shooter was being questioned at police headquarters Tuesday evening. The bicyclist was taken to a hospital in stable condition.
The Port Authority tweeted that downtown bus service was being affected as police remained at the scene.