The 269 is a "connexion route", operating during morning and evening rush hour, taking people between Mackenzie King station in the capital's downtown and Pinehill/Stonehaven station in Kanata.
Twenty-three people were hurt and taken to hospitals and three people died - two on the bus and one on the platform, Watson said.
Police Chief Charles Bordeleau said the bus driver was arrested after the crash, but he declined to provide details on why she was detained.
The Ottawa Police Service Collision Investigation Unit arrived to investigate the incident. Of those transported, 14 were in critical condition and 11 were in serious condition, paramedic chief Myles Cassidy said in the news conference later. He was in his apartment across the street from the station when he started hearing sirens just after 4 p.m.
Authorities responded to the scene and discovered the vehicle "had run into the bus platform at Westboro station", which resulted in numerous injuries on the bus and platform, according to police. He saw first responders cutting out seats and windows from the top floor of the bus and described the front of the bus as being "essentially cut off" or "demolished".
"Our hearts and condolences go out to all those injured, those family members who have lost loved ones and their families", Watson said. "I can't see how they would've been able to get away from that", he said. I felt like vomiting a couple of times.
Ontario Premier Doug Ford took it to Twitter to send a message to the victims and their families. "I have asked staff to ensure all necessary resources are made available to assist them through this hard time".
Brad Muir, a retired sergeant with the Ontario Provincial Police and a collision reconstruction consultant, told CTV News Channel that investigators will be downloading electronic data from the bus and reviewing eyewitness statements, road conditions and video camera surveillance.