A stubborn, six-alarm fire that raged for several hours in a downtown highrise has been extinguished, but around 1,700 residents remain barred from their homes.

Toronto fire officials said that the rare, six-alarm fire was finally put out after a battle that lasted more than eight hours. Seventeen people were sent to the hospital because of the fire.

The injured include three firefighters and three children, according to Toronto Fire Chief Bill Stewart.

"This is probably as ferocious an apartment fire as I've ever seen," Stewart told reporters on Saturday.

He added that as many as 150 firefighters were on the scene at the fire's peak, and that it was actually higher than a six-alarm blaze.

"I think we may have been at seven or eight," he said.

Toronto EMS Chief Bruce Farr said three of the adults suffered "very serious" cases of smoke inhalation.

With daytime temperatures cresting above 30 Celsius yesterday, at least 10 firefighters had to be treated for heat exhaustion.

The Ontario Fire Marshal's Office will now begin the difficult task of investigating the cause of the fire, which started at about 5 p.m. in a unit on the 24th floor.

What isn't clear yet is the integrity of the building's structure. Along with wiring and water damage, there is concern that the fire's intense heat may have damaged the actual structure of the highrise, located at 200 Wellesley St. E.

Even veteran firefighters were surprised by the fire's resiliency, and the fire chief said it was among the hottest blazes he'd seen in nearly four decades on the job.

Chief Stewart added that "extreme fuel loading" in one of the apartment units made the fire very difficult to fight. The heat and high winds yesterday also didn't help.

Meanwhile, many residents were forced to sleep in a community gym Friday night. The damaged building is run by Toronto Community Housing and officials were doing their best to accommodate the residents.

"Our primary concern is the well-being of our tenants," Keiko Nakamura, CEO of Toronto Community Housing, said in a media release. "Our thoughts and concerns are with those tenants and firefighters that have been injured in the fire."

Located in a dense, downtown neighbourhood, the building has more than 700 units. Many residents are considered low income, and they are joined by hundreds of new Canadians. Other residents have physical and mental health issues.

Resident Joanne Blair said the fire has turned her life upside-down.

"I lost everything in my apartment, my cat isn't going to survive," she said at the scene on Saturday.

"You just feel homeless and abandoned because you can't get your stuff. You just leave with the clothes on your back."

Late Friday, seventh-floor resident Saaed Kamal smelled smoke as he fled the building by the stairs.

"I could not breathe very well as I went down the stairs," he said.

Resident Stephen Vassilev believes that the fire began in his apartment, which was packed with documents and files.

"I have nowhere to go," he said.

With a report from CTV Toronto's Michelle Dube