Firefighters were called to the building at 365 Westmount Ave. shortly after 4 p.m. ET.
When they arrived at the scene, they saw heavy smoke coming out of a second-floor window. The fire was quickly deemed to be a two-alarm event.
"We had heavy, heavy black smoke, a lot of heat in all three floors," district fire chief Neil Gallagher told reporters.
Gallagher said people were rescued from both the front and back of the building near Dufferin Street and Rogers Road.
"The tenants had a stovetop fire," he said. "It was a grease fire … they should have put a wet cloth over the fire; instead, they put water on it and it flared up. And when they exited the apartment, they left the door open.
"The fire was very intense in the apartment and it spread down the hallway … and we had the tenants clamouring to get out from the windows because they couldn’t go into the stairwells."
Gallagher said 11 people had to be rescued, a total that was among the highest he could recall in his three decades working as a firefighter.
Paramedics reported that 10 people were transported to hospital to be treated for smoke inhalation.