The bus collided with the platform at the Westboro bus station at 3:50 pm. ET, he said.
Three people died – two on the bus and one person waiting on the platform, Watson said. Twenty-three people were injured and taken to hospitals, he said.
Ottawa Police Chief, Charles Bordeleau, said that many people on the top of the bus had to be freed.
He said investigators are trying to determine the cause of the accident. The driver was arrested and interrogated by the police, but the chief did not provide details.
OC Transpo, who is responsible for Ottawa's public transport, tweeted on his verified account, "The buses are diverted accordingly" of the collision.
The Ottawa Police Service Collision Investigation Unit arrived to investigate the incident. Bordeleau stated that the Ottawa paramedics service and the Ottawa fire services operated in cooperation with the Ottawa police.
The authorities have asked people to avoid the area. Video from CNN's partner CTV News showed several emergency vehicles at the Westboro station.