A cyclist dove into Lake Ontario to try to rescue a driver whose vehicle slammed through a guard rail and plunged from a bridge into the water on Thursday afternoon.

Toronto Police Sgt. Brett Moore said this is now a recovery operation as police divers have just located the submerged vehicle. The car, which witnesses said was being driven by a woman, crashed into the cold water in the Cherry Street and Polson Pier area, in Toronto's east end, around 4 p.m.

The cyclist, who was taken to hospital to be treated for hypothermia, was unable to reach the vehicle as it sank into the nearly eight-metre deep shipping channel, Toronto Fire Platoon Chief Kevin Aucoin said.

Police believe the car smashed through this guard rail and went straight into a shipping channel. (CBC)

Police divers have stopped their work as it's getting dark in the area. The vehicle is set to be removed from the water on Friday morning.

Moore said police believe someone is in the vehicle, but cannot confirm that at this time.

Police investigating cause of crash

Moore said police believe the driver was heading northbound, away from Cherry Beach toward the city, when she slammed through a guard rail on the west side of the bridge and into the water.

Police are investigating what caused the crash. Moore said investigators are reviewing surveillance video from the area and are asking anyone who may have information about the incident to contact them.

Southbound Cherry Street remains closed from Commissioners Street.