The front of the red double-decker bus was sheared off and the engine of the VIA Rail train had derailed, but the train cars remained upright with little noticeable damage.
Ambulances and fire trucks swarmed the scene, and rail officials were helping train passengers off the train.
Ottawa emergency officials said there were six confirmed fatalities in the collision, which occurred at 8:48 a.m. EDT (1248 GMT), and that at least 10 more were critically injured.
VIA Rail, which operates national rail passenger service in Canada, said there were no major injuries reported on the train.
The crash occurred in the rural west end of Ottawa, Canada's capital city, at a level crossing surrounded by corn fields.
Canadian Prime Minister Stephen Harper said he was "deeply saddened" to hear about the collision, which came just months after a runaway freight train crash and explosion killed 50 people in Lac-Megantic, Quebec.
Canada's two big railroads - Canadian National Railway Co and Canadian Pacific Railway Ltd - are reviewing safety standards after the July 6 Lac-Megantic crash that destroyed the center of the small Quebec town.
MNA
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