Police Chief Charles Bordeleau said the bus driver was arrested after the crash, but he declined to provide details on why she was detained. He said police were still investigating the cause of the crash.

"Something led us to having to arrest the individual and take them to the station," Bordeleau said.

The crash took place west of downtown at 3:50 p.m., just as dusk was beginning to settle on the coldest day of the winter so far in Canada's capital

Two of the dead were riding the bus and one was on the platform at Westboro station, a major stop on the city's busway, Mayor Jim Watson said.

The vehicle apparently jumped the curb as it approached the station and plowed along the platform, where the roof of a shelter carved deep into the vehicle's second deck. The first seats on the upper level of the bus were crushed together.

Paramedics said a total of 25 people were taken to the hospital, suggesting one of the victims could not be revived at the scene and the other two died after arriving. Of those transported, 14 were in critical condition and 11 were in serious condition, paramedic chief Myles Cassidy said at a news conference.

"I want to send my deepest condolences to the families of victims and everyone affected by today's tragic bus crash," Prime Minister Justin Trudeau tweeted. "Thank you to the first responders who rushed to help — we're monitoring the situation closely."

The bus had a capacity of 90 people, both sitting and standing.

The crash snarled afternoon traffic as emergency crews descended on the scene.

In 2013, another Ottawa double-decker city bus broke through a warning gate and collided with a Via Rail train in suburban Barrhaven, killing six people as the train sheared off the front of the vehicle.