At least six people have died after a huge fire raged through the night at a west London tower block, and police expect that number to rise.
Eyewitnesses described people trapped in the burning Grenfell Tower, in North Kensington, screaming for help and yelling for their children to be saved.
Firefighters rescued many people and are still trying to put the fire out in the 24-storey block 12 hours on.
Police say there may still be people in the building who are unaccounted for.
During the night, eyewitnesses said they saw lights - thought to be mobile phones or torches - flashing at the top of the block of flats, and trapped residents coming to their windows - some holding children.
It is understood that "several hundred" people would have been in the block when the fire broke out shortly after midnight, most of them sleeping.
Commander Stuart Cundy, of the Metropolitan Police, said the recovery operation would be "complex and lengthy", and the number of fatalities was expected to rise.
He declined to give any details of the number of people who may be missing.
He said it was likely to be some time before police could identify the victims, adding that it was too early to speculate on the cause of the fire.
An emergency number - 0800 0961 233 - has been set up for anyone concerned about friends or family.
More than 70 people have received treatment in hospital. At least 20 are known to be in a critical condition.
By mid-morning, the building looked to be just smoking ruins but the fire has again taken hold, and cladding is falling to the ground.
At 13:00 BST, Fire Commissioner Dany Cotton said firefighters expected to be on the scene for at least another 24 hours and she would not speculate about the cause of the blaze.
She said there were concerns that people were still inside the tower and she urged all residents to make sure they had reported themselves to police so that the authorities know they are safe.
Prime Minister Theresa May is "deeply saddened by the tragic loss of life", said Downing Street.
Meanwhile, Labour leader Jeremy Corbyn is to demand a government statement in Parliament on Thursday on the tragedy, the BBC understands.
Later, police and fire minister Nick Hurd will chair a cross-party meeting to look at how the government can assist the emergency services and local authorities.
Paul Munakr, who lives on the seventh floor, managed to escape.
"As I was going down the stairs, there were firefighters, truly amazing firefighters that were actually going upstairs, to the fire, trying to get as many people out the building as possible," he told the BBC.
He said he was alerted to the fire not by fire alarms but by people on the street below, shouting "don't jump, don't jump".
Eyewitness Jody Martin said: "I watched one person falling out, I watched another woman holding her baby out the window... hearing screams.
"I was yelling at everyone to get down and they were saying 'We can't leave our apartments, the smoke is too bad on the corridors.'"
Michael Paramasivan, who lives on the seventh floor with his girlfriend and young daughter, said he ignored official advice to stay in your home.
"If we had stayed in that flat, we would've perished. My gut instinct told me just to get the girls out. I wrapped the little one up because of the smoke and I just got them out."
Another resident, Zoe, who lives on the fourth floor, said she was woken by a neighbour banging on her door.
"The whole landing was thick with smoke. The smoke alarms weren't going off but the way it spread so quickly from the fourth floor, all the way up to the 23rd floor was scary."
Another woman, Sorsan, is trying to find her mother and other relatives who lived on the 22nd floor.
She spoke to them on the phone after the fire broke out, but hasn't been able to get hold of them since.
"They was screaming and I was saying to them 'get out of it, try and find the nearest exit'," she told BBC Radio 5 live.
"But the police were telling them to put a cover down near the door. I was shouting my head off, just saying: 'run'".
Grenfell Tower, built in 1974, is part of the Lancaster West Estate, a sprawling inner-city social housing complex of almost 1,000 homes.
Robert Black, chief executive of the tower's management company, the Kensington and Chelsea Tenant Management Organisation, said: "The fire at Grenfell Tower is devastating and the reports of injury and losses of life absolutely heartbreaking.
"Along with my colleagues, I have been supporting residents since the early hours, working with the emergency services and the community."
The BBC's Andy Moore, who was at the scene through the night, described watching debris falling from the building, and hearing explosions and breaking glass.
The London Fire Brigade said a structural engineer had checked the building and determined it was not in danger of collapse and that rescue teams were safe to be inside.
Initially, it was feared that the building, which appears to be gutted, could collapse.
Mayor Sadiq Khan said he was devastated by the horrific scenes, attended by more than 250 firefighters and 100 ambulance medics.
Questions will need to be answered over the safety of tower blocks, he told BBC Radio.
"We can't have a situation where people's safety is put at risk because of bad advice being given or if it is the case, as has been alleged, of tower blocks not being properly serviced or maintained," he said.
Matt Wrack, of the Fire Brigades Union said something had clearly gone badly wrong with fire prevention procedures at the building.
Firefighters would normally fight a fire in a tower block from the inside, going up the fire escape, and fighting using the internal dry-rising mains, he said, but that's not been possible in this case.
Construction firm Rydon said recent building work which it carried out on the block "met all required building control, fire regulation and health and safety standards".
Appeals are being made on social media for news of missing friends and relatives, who might have been caught in the blaze.
Emergency rest centres have opened for those now homeless at Latymer Community Centre, St Clement's Church, Harrow Club and Rugby Portobello Trust. There are also local collections under way for spare clothes, toys, blankets and toiletries.
People are being advised by police to stay away from the area, where roads remain closed and nearby residents have been evacuated as a precaution.
Grenfell Tower underwent a two-year £10m refurbishment as part of a wider transformation of the estate, that was completed last year.
Work included new exterior cladding and a communal heating system.
The 24-storey tower, containing about 120 flats, is managed by the Kensington and Chelsea Tenant Management Organisation (KCTMO) on behalf of the council.
Before and during the refurbishment, the local Grenfell Action Group claimed that the block constituted a fire risk and residents warned that site access for emergency vehicles was "severely restricted".
Construction firm Rydon, which carried out the refurbishment, said it was "shocked to hear of the devastating fire" and added that the work "met all required building control, fire regulation and health and safety standards".
Council leader Nick Paget-Brown said the buildings were regularly inspected, but a "thorough investigation" was needed.