Ottawa fire department spokesman Marc Messier told broadcaster CTV that at least five bus passengers died and up to six others were injured.
The Via Rail company operating the affected train on the Ottawa to Toronto line -- on which traffic has been suspended -- reported no fatalities.
The OC Transpo bus was on its way downtown during the morning rush hour when the accident happened, according to local media reports.
The injured were being transported to area hospitals.
Canadian broadcasters showed images of the front of the bus sheered off and the train derailed.
Witnesses, meanwhile, recounted bus passengers being thrown from the bus. (AFP)
