The fire started just before midnight on Tuesday (1700 GMT) in the parking floor of the 10-storey building, witnesses said, an area packed with residents' motorbikes.
"Authorities have rescued about 70 people, rushing to hospital 54 people, including dozens of dead," the official Vietnam News Agency said.
"This is a very serious fire," the statement said.
Photos by AFP photographers on the scene showed flames and smoke billowing from barred balconies.
The blaze was out by Wednesday morning, with rescuers struggling to access the building -- which is down a narrow alley in a residential area of southwest Hanoi.
The building's small balconies were surrounded by iron, with the apartment block having only a single exit -- and no emergency ladder on the outside.
Around 150 people lived in the complex, authorities said.
"I heard a lot of shouts for help. We could not help them much," a woman, who gave only one name Hoa and lives near the block, told AFP at the site.
"The apartment is so closed with no escape route, impossible for the victims to get out."
Another witness, Huong, said a small child was thrown from a high floor to escape the flames.
"I was about to sleep when I smelled something. I went outside and saw the fire," she said.
"The smoke was everywhere. There was a little boy thrown from a high floor, I don't know whether he survived or not although people used a mattress to catch him," she said.
The deputy prime minister, Tran Luu Quang, visited the scene Wednesday morning, state media said.
Vietnam has experienced several deadly fires in recent years, frequently at entertainment venues like popular karaoke bars.
A year ago, a blaze in a three-storey karaoke bar in commercial hub Ho Chi Minh City killed 32 people.
As many as 17 people were also injured in that fire, with the owner arrested on charges related to breaching fire prevention regulations.
In the aftermath of the tragedy, the prime minister ordered an inspection of all high-risk venues.
Thirteen people died in 2018 after an apartment complex, also in Ho Chin Minh City, caught fire. Another 13 died in 2016 in a karaoke venue in Hanoi following a fire.
Southeast Asia frequently sees deadly fires with accidental conflagrations common, a result of lax safety standards.
