Emergency responders were called to the scene just after 8 p.m. Fourteen fire trucks and 55 firefighters were part of the response.
As of 9 p.m., the Toronto Fire Service reported that the fire had been knocked down.
Capt. Mike Strapko told CBC News in a telephone interview that a deceased person had been found in the basement.
Strapko said the fire marshal is being called to the scene.
There is no immediate word on the cause of the fire.
Initial reports suggested that the fire was believed to have started in the basement.
Police say that Gerrard Street is currently closed between Hiawtha Road and Greenwood Avenue.