At least 94 people have died and scores more are missing after the blaze engulfed the Wang Fuk Court housing complex in the northern district of Tai Po.
The fire began on Wednesday afternoon (local time).
Soon after dawn on Friday, firefighters had mostly contained the blaze. The eight-tower estate housing more than 4600 people had been undergoing renovations and was wrapped in flammable bamboo scaffolding and green mesh.
Police said they had arrested three construction company officials on suspicion of manslaughter for using unsafe materials, including flammable foam boards blocking windows.
Firefighters said they expect a search and rescue operation at the still-smouldering complex to be completed by 9am (local time).
"We'll endeavour to effect forcible entry to all the units of the seven buildings, so as to ensure there are no other possible casualties," Deputy Fire Services Director Derek Chan told reporters.
As many as 279 people were listed as missing in the early hours of Thursday morning, but that figure has not been updated for more than 24 hours. Chan said 25 calls for help to the Fire Department remain unresolved, including three in recent hours which would be prioritised.
Rescuers battled intense heat, thick smoke and collapsing scaffolding and debris as they fought to reach residents feared trapped on the upper floors of the complex.
A distraught woman carrying her daughter's graduation photograph searched for her child outside a shelter, one of eight that authorities said are housing 900 residents.
"She and her father are still not out yet," said the 52-year-old, who gave only her surname, Ng, as she sobbed. "They didn’t have water to save our building."
Most of the victims were found in two towers in the complex, while firefighters found survivors in several buildings, Chan said, but gave no further details.
The confirmed death toll rose to 94 early on Friday, the Hospital Authority said. It is Hong Kong's deadliest fire since 1948, when 176 people died in a warehouse blaze.
Police arrested two directors and an engineering consultant of Prestige Construction, a firm contracted to perform maintenance on the buildings.
"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," Police Superintendent Eileen Chung said.
Prestige did not answer repeated calls for comment.
Police seized bidding documents, a list of employees, 14 computers and three mobile phones in a raid of the company's office, the government added.
In a telegram to Hong Kong's bishop, Cardinal Stephen Chow Sau-yan, Pope Leo sent "spiritual solidarity to all those suffering from the effects of this calamity, especially the injured and the families who grieve".
Hong Kong's leader, John Lee, said the government would set up a $HK300 million ($NZ67 million) fund to help residents while companies including automakers Xiaomi, Xpeng and Geely as well as the charity foundation of Alibaba's founder Jack Ma and Tencent announced donations.
On the second night after the blaze, dozens of evacuees set up mattresses in a nearby mall, with many saying official evacuation centres should be saved for those with greater need.
People - from elderly residents to schoolchildren - wrapped themselves in duvets and huddled in tents outside a McDonald's restaurant and convenience shops as a steady stream of volunteers handed out snacks and toiletries.
The eight blocks of the tightly-packed complex of 2000 apartments were home to more than 4600 people in the financial hub, which is struggling to overcome chronic shortages of affordable housing.
Police also said they found foam material sealing windows on one unaffected building, installed in the year-long maintenance work.
The city's development bureau has discussed gradually replacing bamboo scaffolding with metal scaffolding as a safety measure.
China's President Xi Jinping urged an "all-out effort" to extinguish the fire and minimise casualties and losses, state broadcaster CCTV said.
The leadership of both the Hong Kong government and China's Communist Party moved quickly to show they attached utmost importance to a tragedy seen as a potential test of Beijing's grip on the semi-autonomous region.
Hong Kong's sky-high property prices have long been a trigger for discontent and the tragedy could stoke resentment towards authorities despite efforts to tighten political and national security control.
The fire has prompted comparisons to London's Grenfell Tower inferno, which killed 72 people 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.
Hong Kong, one of the world's most densely populated cities, is scattered with high-rise housing complexes. Tai Po, near the border with mainland China, is an established suburban district and home to about 300,000.
Occupied since 1983, the complex is under a government-subsidised home ownership scheme, according to property agency websites, a lifeline for the city's middle-income families.
