York Regional Police say they were called to a collision at Bathurst Street and Clark Avenue West shortly after 10 a.m.
Paramedics initially reported dealing with nine patients, one of whom was airlifted to a Toronto hospital. Later on, paramedics said that six people were taken to hospital, in addition to the critically injured man.
An Ornge air ambulance spokesperson told CBC News that the critically injured patient is a 20-year-old man.
Police say Bathurst Street will be closed in both directions for several hours, as will the westbound lanes of Clark Avenue West.