Ontario's police watchdog is now investigating a crash that sent four people to hospital after a car collided with an OC Transpo bus in downtown Ottawa following a police pursuit.

Ottawa paramedics responded to the crash just after noon on Thursday at the intersection of Laurier Avenue East and Nicholas Street.

Paramedics assessed and treated four people, including the drivers of the car and bus, plus two passengers.

The 59-year-old car driver and 48-year-old bus driver, both men, suffered neck and back injuries and were taken to hospital in stable condition.

A 39-year-old car passenger, also a man, suffered a fractured hip and shoulder and was taken to hospital in serious but stable condition.

A 32-year-old woman who was on the bus suffered minor injuries, but she was taken to hospital as a precaution because she is pregnant.

Ottawa police confirmed they were pursuing the vehicle before the crash and as a result, the Ontario Special Investigations Unit has invoked its mandate.

The SIU investigates all cases when police officers are involved in incidents where someone has been seriously injured, dies or alleges sexual assault.