Officials said the remains of three victims had been recovered and some 30 more were unaccounted for, while the local fire chief said rescuers were now searching for bodies.
The blaze at the home, which housed around 50 to 60 elderly people in 52 units, broke out shortly after midnight.
"We could hear screaming from inside. The fire was intense, it was like a haystack on fire," Pascal Fillion, a neighbor who witnessed it, told French-language public broadcaster Radio-Canada.
The home is in L'Isle-Verte, a small town 450 kilometres northeast of Montreal with a population of around 1,400 people.
The town's acting mayor, Ginette Caron, told a news conference that most residents of the home are reliant on caregivers.
She said these include elderly people needing "100 percent care, almost all in wheelchairs, using walkers, or who aren't mobile at all, people suffering from the late stages of Alzheimers.
"The types of services offered here are not found just anywhere. That's what we've also lost," she said.
Two firefighters were also hurt.
Fire chief Yvon Charron said that a third of the building had been evacuated and his crew hopes to breach the remaining areas worst hit by the flames by the end of the day "to search for any bodies."
The Red Cross set up a makeshift shelter at a local school where several people rescued from the inferno spent the night, according to a representative, Myriam Marotte.
Some residents might have been away visiting family, or may have taken refuge elsewhere during the blaze and missed being counted, she told local television.
Images broadcast on local television showed flames engulfing the wood-frame building, leaving only a solitary chimney standing by morning.
