The bus hit a shelter in wintry conditions at the start of the afternoon rush hour in the Canadian capital. There were multiple injuries on the crowded bus, some of them fatal.
The westbound 269 bus to Kanata hit the shelter at the busy transit station in the west end of Ottawa just before 4 p.m. local time (2100 UTC) on Friday.
In cold, wintry conditions, part of the upper right side of the double-decker was removed.
Ottawa Mayor Jim Watson said 23 people were injured in the crash, some of them critically. Three people died; two were believed to have been on the bus, and one person was waiting on the platform. The bus could have been holding as many as 90 passengers.
The city's police chief Charles Bordeleau said the bus driver was arrested at the scene and taken in for questioning. "An investigation is now underway on the cause of the collision," Bordeleau said.
"There's no question that the weather adds complexity at the scene," said Bordeleau said.
It was unclear if weather contributed to the crash. Investigators were set to stay at the scene throughout Friday night, with temperatures expected to drop to minus 22 degrees Celsius (minus 7.6 degrees Fahrenheit) through Saturday.
A passenger who had been on the upper deck of the bus told CBC News that people at the front were pinned under their seats. "There was definitely people pinned. And there was a bit of blood on the floor," Bilal Gill said.
Police were quickly at the scene and broke windows to reach trapped passengers.
Four hours after the crash, Ottawa Hospital said that seven patients were in a critical condition and nine in a stable condition.
Flags at city hall have been lowered to half-mast. Prime Minister Justin Trudeau sent his condolences, and thanked first responders:
The crash is the second involving an Ottawa transit bus in recent years. In 2013, a double-decker commuter bus crashed into a passenger train, killing six people including the driver.
jm/amp (AP, AFP)