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.
Passengers on the train, which was heading to Toronto, said they felt a small impact.
“All I felt was a bump, and I saw a bit of smoke. I thought we were going off the track … I was afraid we were going to flip over,” passenger Robert Gencarelli told reporters on the scene.
He said he was startled when he got off the train and saw how badly the bus was damaged.
“That hit home.”
Another train passenger, who did not give his name, said he saw the bus rolling toward the train tracks and knew the collision was about to happen.
“I saw it before it happened. I was expecting something. There was a big bang. … The bus was rolling. It didn’t stop.”
The crash occurred in the rural west end of Ottawa, Canada’s capital city, at a level crossing surrounded by corn fields.
A reunification center was set up for families and friends looking for passengers on the bus and train, the City of Ottawa said.
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.
“Our thoughts and prayers are (with) the families of those involved,” Harper said on Twitter.
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.