The city's police chief Charles Bordeleau said the bus driver was arrested at the scene and taken in for questioning.
According to the Ottawa Citizen, the bus was travelling west on the Transitway at around 3:50 p.m. on Friday when the crash occurred.
Two of the dead were riding the bus; one was on the platform at Westboro station, a major stop on the city's busway, Watson said.
"The upper deck on the bus was compromised and several people were trapped on the upper level for a period of time", the news release said.
OC Transpo has issued notices saying detours are in effect for the Dominion and Westboro transit stations.
Photos from the scene show that one side of second floor was partially crushed by the roof, and multiple people were being treated next to the bus.
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Sgt. Graham said six investigators are now assigned to the crash, which is expected to be a "long, detailed and complex investigation". "I know all those impacted by this tragedy will be surrounded and supported by many", Bordeleau wrote in a tweet published late Friday evening.
Crash Investigation Unit is continuing its investigation into the collision yesterday, which left 23 people injured and three dead. Of those transported, 14 were in critical condition and 11 were in serious condition.
"Our hearts and condolences go out to all those injured [and] those family members who have lost loved ones", Watson said.
One person was also taken by ambulance to the Queensway Carleton Hospital, although that patient's condition has not yet been made public.
Monica Lung was on her daily bus ride home from Carleton University in Ottawa when she passed the horrific scene at Westboro station, where a double-decker bus jumped the curb and smashed into the transit shelter.
He says the bus was at or near its full capacity of 90 passengers at when it slammed into a bus shelter on Friday afternoon, and police will have to interview everyone on board at the time.
The Ottawa Hospital tweeted Saturday morning that one patient is now under treatment in critical condition, six patients are in serious condition and four patients are in stable condition following Friday's crash.