At least six people have died after a huge fire raged through the night at a west London 24-storey 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, who rescued many people, were called at 00:54 BST and are still trying to put out the fire.
Police say there may still be people in the building who are unaccounted for.
The ambulance service said 68 patients had been taken to six hospitals across London, with 18 in critical care. A further 10 patients made their own way to hospital.
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.
At 13:00 BST, Fire Commissioner Dany Cotton said firefighters expected to be on the scene for at least another 24 hours.
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.
Health Secretary Jeremy Hunt praised the "heroic" response from the emergency services and the NHS hospital staff "working tirelessly to help".
Meanwhile, Labour leader Jeremy Corbyn is to demand a government statement in Parliament on Thursday on the tragedy, the BBC understands.
There must be a "full inquiry" into the fire, newly-elected Kensington MP Emma Dent Coad told the BBC.
Speaking outside the Rugby Portobello Trust emergency centre, the Labour MP said the fire was "absolutely appalling".
"The ferocity of that fire was extraordinary and terrifying," she said. "This must never happen again."
Police and fire minister Nick Hurd was due to 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."
At the scene
By Lucy Manning, BBC News
They have lost their homes and for some, tragically their relatives.
At times there is the sound of sobbing as the word goes round that someone is missing, someone is feared dead.
I've spent the day inside the community centre where survivors have gathered.
Downstairs in the hall families sit at tables and wait for news.
One family told me they hadn't heard from their brother, sister and three children - Mirna, Fatima and Zainnb. Other relatives were out searching hospitals. There was still no news.
Outside the centre, Sawsan was with a group of women. For one it was too much, she was on the floor crying. Sawsan hasn't heard from her mum, sister, brother-in-law and nieces. She spoke to them when the fire started but nothing since.
Inside the centre, families are being helped with food, housing and medical treatment. It's busy and everyone is helping. Just not with the one thing they need - information about whether their relatives are safe.
Christabel told me how lucky her father had been. He tried to fight the fire but made it out alive.
Ed was saved when a friend called him to tell him to get out the building. "I'm lucky" he says. But they have lost everything.
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.
Read more on safety concerns here.