Almost a full day after the fire began, firefighters were struggling to reach residents potentially trapped on the upper floors of the Wang Fuk Court housing complex due to intense heat and thick smoke from the blaze that erupted on Wednesday afternoon.
The tightly packed complex in the northern Tai Po district has 2000 apartments in eight blocks that are home to more than 4600 people in a city struggling with chronic shortages of affordable housing.
Video from the scene showed flames leaping from at least two of the 32-storey towers sheathed in green construction mesh and bamboo scaffolding.
The fire was still burning on Thursday afternoon, more than 24 hours after it started.
Authorities said they had brought the blaze in four of seven blocks under control, with operations continuing in three.
Bamboo scaffolding is a mainstay of traditional Chinese architecture but has been subject to a phase-out in Hong Kong since March for safety reasons.
Police officers searched the housing estate's building maintenance company on Thursday, seizing documents that mentioned Wang Fuk Court.
The company did not immediately respond to a request for comment.
"We have reason to believe that the company's responsible parties were grossly negligent, which led to this accident and caused the fire to spread uncontrollably, resulting in major casualties," said Eileen Chung, a Hong Kong police superintendent.
Three men from the construction company, two directors and one engineering consultant, had been arrested on suspicion of manslaughter over the fire, she said.
Police said that as well as the buildings being covered with protective mesh sheets and plastic that might not meet fire standards, they discovered some windows on one unaffected building were sealed with a foam material, installed by a construction company carrying out year-long maintenance work.
A firefighter was among the 55 killed with dozens in hospital in critical condition, Hong Kong authorities said on Thursday afternoon.
Some 279 people remain uncontactable.
Chung said 900 residents were spread across eight shelters.
The death toll is the highest in a Hong Kong fire since 1948, when 176 people were killed in a warehouse blaze.
"The priority is to extinguish the fire and rescue the residents who are trapped," Hong Kong leader John Lee told reporters.
"The second is to support the injured. The third is to support and recover. Then, we'll launch a thorough investigation."
The fire has prompted comparisons to the Grenfell Tower inferno that killed 72 people in London in 2017.
That fire was blamed on firms fitting the exterior with flammable cladding, as well as failings by the government and the construction industry.
"Our hearts go out to all those affected by the horrific fire in Hong Kong," the Grenfell United survivors' group said on social media.
"To the families, friends and communities, we stand with you. You are not alone."
Many residents took to social media to criticise what they saw as negligence and cost-cutting as a cause of the fire.
One video showed several construction workers smoking on the bamboo scaffolding surrounding one of the complex's blocks during the renovation process.
From the mainland, China's President Xi Jinping urged an "all-out effort" to extinguish the fire and to minimise casualties and losses, China's state broadcaster CCTV said.
Occupied since 1983, the complex has been undergoing renovations for a year at a cost of $HK330 million ($A65.5 million).
Hong Kong's corruption body said it has launched an investigation into suspected corruption related to the renovation.
