Four people were injured in an early morning fire in northwest Toronto on Saturday, paramedics say.

The fire broke out in the basement of a building on Weston Road north of Eglinton Avenue West. Emergency crews were called to the scene at about 2:30 a.m. The building is both commercial and residential.

Toronto paramedics took a woman in her 30s in life-threatening condition and a man in his 50s with burns in serious condition to hospital, Evert Steenge, deputy commander of Toronto Paramedic Services, said on Saturday.

Two other people were taken to hospital with minor injuries.

When firefighters arrived, flames were visible, according to Capt. David Eckerman of Toronto Fire Services. Firefighters found the woman on a stairway and brought her to paramedics.

The man, who suffered burns to his back, went to a nearby bar for help before he was transported.

About 36 firefighters and at least nine trucks responded to the blaze. (Jeremy Cohn/CBC)

Eckerman said the fire migrated from the basement to the underside of the first floor.

"It got into the walls in the basement. It was in and around the furnace area. It did some damage to the floor above," Eckerman said.

Firefighters extinguished the second-alarm fire before 4 a.m. About 36 firefighters and at least nine trucks responded to the blaze.

Ontario's Office of the Fire Marshal is investigating. The cause of the fire is not yet known.

Leianne Musselman, communications director for the Canadian Red Cross, based in Ontario, said trained volunteers are helping two people affected by the fire.

Weston Road, which was closed as firefighters fought the blaze, has been reopened. Firefighters are continuing to maintain a "fire watch" at the scene.