A 39-year-old man is facing four charges after allegedly punching an officer in the face and starting a dramatic foot chase in central Ottawa Saturday afternoon.

Ottawa police said they pulled a black sedan over near the intersection of Bank Street and Heron Road at about 3 p.m. Saturday.

"He [the suspect] became very irate and argumentative," said acting Insp. Sean McDade of the Ottawa police. "At one point he exited the vehicle, became assaultive and punched one of our officers in the face."

The suspect then tried to run through four lanes of traffic, police said. At that point, a police officer who wasn't in uniform appeared on the scene with a gun, according to witness Peter Dickson.

"[The suspect] bolted, he ran around the car and right into traffic on Bank [Street], bumped into the side of a car in the second lane. ... All of a sudden this fellow appears out of nowhere out of this parking lot, he was standing on the grass pointing a pistol at the suspect," Dickson said.

"There was a great big crack, there was a gun fired."

McDade said two Ottawa police officers fired stun guns at the driver, the first one missing but the second one hit its target.

Ottawa police later said the gunshot came from an Ontario Provincial Police officer who wasn't in uniform. It was fired between the first and second stun gun attempt.

Bank Street was closed for more than four hours as police dogs and RCMP explosives disposal officers searched the sedan.

Police are investigating why a shot was fired.

The officer who was punched was taken to hospital with facial injuries.

The suspect is charged with assaulting police, obstructing police, resisting arrest and driving while being suspended from doing so.