Firefighters and other emergency crews are still on the scene of a sauna explosion at a downtown Toronto high rise that sent one firefighter to hospital last night.

The incident happened just before 11 p.m. when Toronto Fire Services says there was a "flashover explosion" at 57 Charles St. Firefighters at the scene called in a mayday, indicating they were in distress.

Fire crews had seen a bit of smoke coming from the sauna in the apartment building, but not enough to realize it was fully engulfed in flames.

"They opened the door, oxygen went in and the fire basically just exploded," said District Chief, James Green. "It was a huge blast, an incredible force."

Fire officials confirmed the explosion threw four firefighters down the hallway of the apartment building.

One firefighter suffered hand burns and was taken to hospital, two others remained on scene with less severe burns.

"It's a huge tragedy averted," said Green. "We're just thanking our lucky stars that nobody was more seriously hurt, civilians and of course our fellas."

The call was a four-alarm incident at one point, but was soon downgraded to a one-alarm "minimal fire."

Toronto police are holding the scene until the Ontario Fire Marshal arrives later this morning.

Toronto Police guarding scene at Charles St fire until arrival of OFM in the morning. TFS crews clearing. <a href="https://t.co/ccCg6bLx0f">pic.twitter.com/ccCg6bLx0f</a> —@LateNightCam

4 <a href="https://twitter.com/hashtag/TorontoFire?src=hash">#TorontoFire</a> Fighters were thrown down the hallway from the explosion. <a href="https://t.co/k8p81NnwMF">https://t.co/k8p81NnwMF</a> —@Toronto_Fire