Firefighters in Mississauga battled a blaze on Sunday that was ignited by a "substantial explosion" that injured three people, caused widespread damage and forced an evacuation of nearby apartments.

Emergency crews first responded to a strip mall in the area of Agnes and Hurontario Streets, near Dundas Street, at 7:28 a.m. after multiple reports of a loud explosion. The mall houses both commercial businesses and private residences.

"There was a huge sound. We all got up," said Sami Mohamed, who lives in a nearby apartment. "My kids were crying."

When firefighters arrived on the scene, they discovered that at least one third of the building had collapsed, and that three people inside a residence next to the blast site had moved to an upper level floor to avoid the flames. Aerial ladders were used to rescue the three from the fire that broke out following the blast.

According to Mississauga fire chief Tim Beckett, staff at a fire station north of the blast site and Mississauga's city manager — who lives nearby — felt the explosion.

Businesses and apartments across the street have smashed windows as a result of the explosion <a href="https://twitter.com/hashtag/mississauga?src=hash&ref_src=twsrc%5Etfw">#mississauga</a> <a href="https://t.co/nmv08ZsMJ9">pic.twitter.com/nmv08ZsMJ9</a> —@TalRicci

Jay Szymanski, spokesperson for Peel paramedics, said the three people were transported to a local hospital from the scene. A 39-year-old Mississauga man suffered serious but non-life threatening injuries, while a 43-year-old woman and a six-year-old boy were both taken for treatment of minor injuries.

Two other people, a 50 year-old man and a young child, who had been walking in the vicinity of the strip mall when the explosion went off were treated by paramedics at the scene but did not require transport to hospital.

The building is significantly damaged. Firefighters struggled to knock down the blaze because the blast compromised a natural gas line, Beckett said. Enbridge Gas crews had to excavate a section of ground before the line could be shut down. Freezing rain also complicated the situation for fire crews.

Residents in the area reported hearing and feeling an explosion shortly before 7:30 a.m. Sunday. (CBC)

"This is going to be a long event," Beckett said.

By early Monday morning, firefighters had ceased using water cannons and the blaze appeared to be out.

The initial explosion shattered glass windows in storefronts across the street and structurally damaged a nearby apartment building. Some residents have been forced out of their homes.

"A number of families have been displaced from the area of the explosion and it is uncertain when they will be allowed back into their homes," police said in a news release.

"To assist these families, Region of Peel Human Services is currently leading a coordinated response with the Red Cross at the evacuation centre at Mississauga Valley Community Centre."

Ontario's Office of the Fire Marshal is working with fire investigators to determine what may have caused the explosion. According to Beckett, crews will likely not reach the epicentre of the explosion until some point on Monday.

Hurontario Street is still closed in both directions from Dundas Street to Hillcrest Avenue as officers investigate. Police are asking motorists and pedestrians to avoid the area until the scene has been fully secured.