Between 200 and 300 people rushed out of their homes before sunrise, some in just bathrobes and pyjamas, after the blaze, which is believed to have started in the kitchen of one unit, spread quickly to neighbouring homes.
"It was a very intense fire," said Peel Region Police Const. Lillian Fitzpatrick. "These people had to leave in the very early morning hours with nothing but the clothes on their backs."
While almost all affected residents were able to escape the leaping flames, police said firefighters found the body of a young boy in the home where the fire is believe to have started.
The child was at the home for a sleepover, Fitzpatrick said, but his family lives nearby and had to leave their own home in the middle of the night.
"Obviously they are completely devastated at this point," she said of the boy's family.
"It's a very, very fresh wound for them. They need some time to collect themselves and deal with this horrible blow."
Police said an entire block was affected by the fire, which broke out just after 3 a.m.
"The fire started in one unit that was in the middle of that block and very quickly spread to the units on either side," Fitzpatrick explained.
"I had one person that was evacuated tell me that the flames were so hot and so bright that they couldn't look at it and that there was just massive amounts of smoke."
It was the quick action of those living in the complex that likely saved many lives, Fitzpatrick said.
"You've got the smoke, you've got the heat, you've got the flames and people were running to neighbours, banging on windows and banging on doors," she said.
"At that hour in the morning people would have been sound asleep. If these people hadn't gone and woken their neighbours and passed that information along, it's very possible that there could have been definitely more injuries and possibly more fatalities."
Eighteen homes were "virtually destroyed" in the blaze, while fire officials still must assess the structural integrity of a number of other units, police said.
By midday, the fire had been brought under control, but police said it was "still active" in certain hot spots with smoke continuing to linger. Fire crews remained on scene, using several ladders to get to smoldering, blacked roofs.
Those forced out of their homes were being helped by Peel Region social services, the Salvation Army and the Red Cross.
Temporary accommodation was being arranged for those who needed it and an online fund was set up for those wishing to donate to help residents affected by the fire.
"The way this community here has pulled together is awesome," said Fitzpatrick, who noted that there had already been an outpouring of assistance for those affected.
Officials said it could still be a day or two before people are allowed to return to homes that survived the blaze.
"It's very difficult to say when they'll be able to go back because for them to go back to their homes it has to be safe for them to do so," said Fitzpatrick.
"There are going to be immediate needs and social services are going to see that they are taken care of."
The Ontario Fire Marshal’s office, Peel police and the coroner's office are all investigating the incident.