A six-alarm fire at a waterfront Toronto recycling plant is finally under control after the inferno burned for hours, caused major structural damage and sent smoke drifting deep into the city.

Toronto Fire Chief Matthew Pegg announced the fire was technically under control shortly before noon on Thursday, although crews are expected to remain on scene throughout the day and possibly into the night in order to fully extinguish hot spots.

The building is a solid waste transfer station run by Green For Life Environmental Inc.

The company confirmed on Thursday that no employees were injured and the fire won't disrupt waste collection.

After hours of firefighting, crews were finally able to overcome the fire, but the roof collapsed and there was tremendous damage to the structure, officials said. (Fire Chief Matthew Pegg/Twitter)

"There will be no interruption of service to curbside services for city residents. Commercial and industrial collection today may be delayed but we anticipate will be back on schedule tonight and tomorrow," a statement from the company said.

The blaze began just before 1 a.m. at the Cherry Street plant and quickly escalated to a six-alarm fire at its peak.

Speaking earlier on CBC Radio's Metro Morning, Toronto Fire District Chief Stephan Powell said the fire could take days to finally extinguish.

"There's a lot of combustible material."

The blaze eventually became a six-alarm fire, with crews using water from an emergency firefighting vessel docked nearby to help in the effort. (CBC)

By afternoon, heavy equipment was brought in to remove parts of the building and recycling material so firefighters could access the areas that were still burning, Pegg said.

"The content is far too heavy for our firefighters to move manually — we need forklifts and backhoes and such to pull it apart safely," he said, adding that it was hard to estimate just how long it would take to entirely put out the flames.

"It really depends how this process of taking it apart and fully suppressing it goes."

A steady rain fell as the fire burned and Pegg said that had helped prevent any embers from flying around.

Fire prompted roof collapse

The cause of the fire remains unclear.

Crews have been able to identify an area of interest inside the building, but the investigation is moving slowly due to the damage caused by the fire, Division Chief Larry Cocco said.

"There's extensive damage in the structure. The roof has collapsed."

More than 100 Toronto firefighters and at least 50 vehicles were sent to the six-alarm fire. (CBC)

"We will be continuing to excavate the area of interest once the fire has been extinguished."

Cocco said witnesses reported seeing fireworks near the recycling plant around the time the fire began.

But after further investigation, firefighters have determined the fireworks were north of the recycling plant and it's considered unlikely that they were the cause of the blaze.

Smoke across city

​The fire sent thick smoke plumes high into the air. People could smell smoke as far away as Etobicoke, said Pegg.

While officials said there was very likely no possibility of harm to the public, the air in the area was monitored for dangerous substances. (Peter Valkov/CBC) Toronto Mayor John Tory visited the scene on Thursday morning and told reporters the smoke is not a health concern.

"​People don't need to worry about that. There is no good smoke, but this is smoke people just smell widespread across the city and it doesn't, as best as anyone knows at the moment, pose any danger," the mayor said.

But residents said the smell was thick and inescapable for hours.

"I got to work at about 7 a.m. at Dixie and Bloor and I could smell it," Connie Shepherd told CBC Toronto. "I could even smell it at home in Liberty Village, too," adding that she actually checked her apartment to see if something was burning.

Unclear what's burning

The fire chief told reporters that because the plant is a mixed waste and recycling facility crews are unsure of what's burning inside.

"It's really hard, if not impossible, to know what the contents are."

In its statement, GFL Environmental Inc. said no hazardous materials are on site.

The fire in the Portlands could be smelt across the city, as prevailing winds pushed smoke right over the city. (CBC)

Pegg said much of the water being used to battle the blaze was being supplied by a fireboat just offshore in Lake Ontario.

GFL Environmental Inc. describes the facility on its website as a solid waste transfer station, hauling yard and solid waste material recycling site.