It took 100 firefighters more than four hours to put out a blaze that gutted a popular downtown Toronto computer store early Monday morning.

A passerby placed an emergency call just before midnight to report black smoke billowing from Canada Computers on College Street west of Spadina, Toronto Fire Services said.

Two firefighters were taken to hospital with minor injuries suffered fighting a blaze that gutted a Canada Computers store on College Street. (CBC)

Firefighters from six stations arrived to thick smoke and intense heat emanating from the four-storey building.

Residents safely evacuated upstairs apartments before the flames shattered their windows and devastated the main-floor store.

"There was some significant damage to the first floor, and at one point the floor caved in," District Chief Dan Sell said. "We mounted an aggressive defensive attack using devices such as our aerial tower and monitors to spray large bombs of water in."

Fire crews needed four hours to quench the flames, which spread quickly because of the building's old structure, the fire department said.

Two firefighters were taken to hospital with minor injuries. Adjacent stores and apartments suffered extensive smoke damage.

Toronto Fire Services said the blaze began in the basement, but officials are still looking into how it started.

College Street was closed between Spadina Avenue and Bathurst Street on Monday morning.