With at least 94 people killed, the inferno at Wang Fuk Court is Hong Kong's deadliest fire in more than 70 years.
Authorities say the "negligent" use of flammable building materials contributed to the rapid spread of the blaze across a group of residential high-rise apartments.
In a city with one of the highest population densities in the world, about 4,800 residents lived in towers, including many older people.
There are fears the death toll could rise significantly with more than 250 people still missing and dozens of others hospitalised.
This is how the catastrophe unfolded.
The first call came in at about 2:52pm on Wednesday, local time, that a high-rise building in Hong Kong's Tai Po district was on fire.
In a cluster of eight 32-storey residential towers, it was not long before the buildings on either side were ablaze.
At 3:34pm, a category three fire alarm was raised as residents frantically fled the Wang Fuk Court apartments.
The densely populated district is home to more than 300,000 people, and the towers house about 2,000 apartments.
ABC journalist Mary Lloyd, who used to live in Hong Kong, said the area was once a fishing village but over the past few decades, thousands of flats were built in densely packed estates.
"The area is surrounded by country park and is served by the train line that connects the central districts of Hong Kong with the border of mainland China," she said.
By 5pm, large crowds had started gathering, according to local press.
People were crying and folding their hands in prayer, watching as more and more apartment blocks were engulfed in flames.
Four people had already been declared dead.
A 37-year-old firefighter who was rushed to hospital died at 4:41pm.
Thick smoke billowed and loud cracking sounds were heard, possibly from the burning bamboo scaffolding surrounding the buildings, which had been under renovation.
Just before 6:30pm, a level five fire alarm was raised, the highest level available.
Hong Kong's fire department said it expected the fire to be extinguished by Friday, more than 24 hours after it started.
It was the first five-alarm fire in Hong Kong since the Cornwall Court fire in 2008, when four people died after a blaze broke out in a nightclub and karaoke bar in Mong Kok.
It is also now Hong Kong's deadliest fire since 1948, when a fire at a five-storey warehouse in the Central and Western district killed 176 people.
How the fire spread so rapidly
The fire rapidly spread to seven of the eight towers at a pace authorities said was "unusual".
Fire Services director Andy Yeung Yan-kin revealed that officers discovered styrofoam inside the burning buildings.
Styrofoam boards had been used to block windows in some of the apartments.
Mr Yeung said the "highly flammable" material caused the fire to spread more quickly within the blocks and ignite flats through the corridors.
While seven of the towers burned, police and firefighters were able to access the eighth building.
Security Secretary Chris Tang said they discovered "exterior netting, tarpaulin and plastic sheeting that burned far more intensely than permitted materials", as well as styrofoam stuck to window frames.
Two directors and a consultant for a construction company have been arrested on suspicion of manslaughter, Hong Kong police have confirmed.
Occupied since 1983, the complex is under the government's subsidised home ownership scheme, according to property agency websites.
Last year, it made headlines after a $HK330 million ($65.37 million) renovation was approved.
The construction works have seen the buildings encased in bamboo scaffolding and green netting, which fanned the fire and worsened conditions for firefighters.
Footage shows the fire spreading quickly through the bamboo.
Ehsan Noroozinejad, a senior researcher and global challenge lead from the Urban Transformations Research Centre at Western Sydney University, said the bamboo scaffolding acted as a continuous external fuel path.
"Dry culms [bamboo stems] have high surface-area-to-mass ratios and, when wrapped with plastic mesh, create a ventilated 'chimney' that supports rapid flame spread and radiant pre-heating of the next bay," he said.
"The lattice continuity, closely spaced ledgers and lashings provided uninterrupted heat bridges across floors and corners; once windows failed, the exterior fire coupled with interior contents, accelerating the event.
"This isn't a craftsmanship issue but a materials-and-configuration problem: a combustible, porous frame placed against an occupied high-rise with few intentional breaks."
For decades in the skyscraper-strewn city, bamboo has been the material of choice for scaffolding — cheap, abundant and flexible — bound together with nylon cords.
The government began phasing out the use of bamboo in March for safety reasons.
Firefighting and evacuation challenges
Fire Services deputy director Derek Armstrong Chan said falling debris and scaffolding posed "additional danger to our front-line personnel".
Another major difficulty faced during the operation was the number of floors on fire, authorities said.
Officers were only able to enter the buildings from the ground floor during the rescue.
Firefighters worked through the night, but struggled to reach the upper floors of the 32-storey towers due to intense heat and thick smoke.
Local media reported that water jets also did not seem to be able to reach the higher flats.
More than 750 firefighters, 120 fire engines, 50 ambulances and about 400 police officers responded to the blaze.
Raymond Cheung, a former firefighter from Hong Kong who now lives in Heidelberg in Melbourne, knows firsthand how challenging the disaster would have been for his former colleagues.
"It's so tough with high temperatures, poor ventilation, and they have to carry so much equipment," he told ABC Radio Melbourne.
"They're still doing a very hardworking and industrious job, so it's time to pray for them, and to support them as well."
Fears for missing residents
A 65-year-old resident surnamed Yuen said the neighbourhood was home to many older residents who use wheelchairs and walkers.
He said as the apartment complex was undergoing maintenance, many residents kept their windows shut and so did not hear the fire alarm.
"There is loss of property and loss of life, and even a firefighter has died," Mr Yuen told the AFP news agency.
The blaze left him and his wife homeless.
Ng's windows were locked when she heard what she thought were firecrackers.
The woman, who declined to give her full name, managed to flee her 19th-floor apartment.
Ng, in her 60s, said her heart was pounding as she and her family hurriedly evacuated the home she had lived in for more than four decades.
More than 900 evacuees made it to shelters, but hundreds more were still missing on Thursday.
Police said they had received calls earlier from residents still trapped in the building.
Residents have been documenting missing persons through a social media app.
It shows reports submitted through a linked Google document about individual towers and rooms.
It includes descriptions such as "Mother-in-law in her 70s, missing" or "one boy and one girl" or "a middle-aged lady, weak" or "Rooftop: 33-year-old male."
Most of the missing reports are from the upper floors of the "E" and "F" tower blocks.
Hundreds of pets trapped
Hong Kong resident Jason Kong was forced to leave behind his 10-year-old poodle, Ber Ber, when police barred him from returning to his apartment.
"My dog was at home, so I thought of coming back and bringing the dog down from the apartment. It was just around 5pm and the police would not let me up," he said.
"I am afraid the smoke will suffocate him."
A pet rescue organisation estimated that more than 100 pets were trapped inside the estate.
"We estimate that every block has more than 10 animals of different species," Zoie Cheng Kam-shan, business development director of the Hong Kong Pet Club, told South China Morning Post.
"We have received over 70 reports but are receiving more. So I estimate that there will be more than 100."
By early Thursday morning, local time, authorities said they had brought the fire in four blocks under control, with operations continuing in three blocks after more than 15 hours.
The latest 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.
"The priority is to extinguish the fire and rescue the residents who are trapped," Hong Kong leader John Lee told reporters earlier.
"The second is to support the injured. The third is to support and recover. Then, we'll launch a thorough investigation."
ABC/Wires
