Emergency crews responded to Bathurst Street and Fort York Avenue just after 11pm after reports of an explosion under the bridge.
Video posted on social media showed loud fiery explosions under the bridge and thick smoke billowing from the area.
Toronto Fire said a fire was extinguished at the scene, but did not say exactly what caught fire or how it started. They said there is no confirmed evidence of any type of explosion involving fuel or propane tanks at this time.
They said investigators with Toronto Fire will investigate the incident.
The busy bridge is right next to the Fort York library and several large apartment towers.
Bathurst Street was closed for much of the night but reopened around 6:30am