Hospital authorities said the blaze late on Thursday was thought to have been caused by a short circuit in a generator, although the city mayor said sabotage could not be ruled out.
Firefighters tackled the fire at the private Badim hospital, near Rio's Maracana World Cup football stadium, for several hours before finally managing to extinguish it.
The city's forensic institute said most of the dead were aged 66 or over, many of whom were in the intensive care unit.
"The majority were due to suffocation and other causes related to the accident... the devices keeping them alive stopped working because of the fire," Gabriela Graca, director of the forensic institute, told local media.
Shehla Raza’s office catches fire
Emergency personnel searched through the burned-out building until the early hours of Friday to recover bodies.
The hospital said there had been 103 patients in the building when the fire broke out and that "more than 100 doctors were mobilized to bring help to the victims." Seventy-seven of them had been moved to other hospitals while 14 had been cleared to return home.
During the evacuation, patients on gurneys and stretchers were carried into surrounding streets as ambulances struggled through crowds of curious onlookers to transport patients to other medical facilities.
"The doctor arrived in the room and told us that there was a fire and that we had to evacuate as quickly as possible," 58-year-old Teresa Dias, who was visiting her father, told AFP.
Fire erupts in medical college warehouse
"They put him in a wheelchair and tied him up to prevent him from falling out. There were a lot of other sick people on stretchers on the stairs."
Staff wheeled medical equipment outside, and the most seriously ill patients were taken at first to a nearby children's nursery. Sheets tied together were seen hanging from hospital windows.
"I was able to take my mother out of her room and when we got to the fire escape, there were a lot of people running around," lawyer Carlos Otorelo, whose 93-year-old mother was being treated for pneumonia, told the UOL news website.
"It was terrible because the smoke spread very quickly."
Other relatives were forced to wait outside for word on their loved ones as dense black fumes filled the sky.
"I heard the loud noise of glass breaking and thought it was shooting or a robbery when I heard screaming. I went downstairs to see what was happening and I saw a lot of smoke," said one resident, Terezinha Machado, 76.
Labour safety remains lax seven years after Baldia factory fire
Hospital authorities said smoke spread to all floors of the building and pointed to a generator in the oldest part of the complex as the probable cause of the fire.
Mayor Marcelo Crivella said investigators would probe whether the fire was an accident.
"Experts will have to find whether anyone was responsible. I hope I am wrong, but we have to check there was no sabotage, we have to investigate," he told reporters when visiting the hospital on Friday.
The fire service said the premises had safety certificates.
Rio has been hit by two major fires in the past year, including when the National Museum was gutted by flames last September, destroying most of its priceless collections.
In February, 10 teenage members of the Flamengo football club were killed in the prefabricated building where they were staying.
Faulty air conditioning systems were the cause of both fires.
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ