Several people have died after a huge fire raged through the night at a west London tower block, a fire chief says.
Eyewitnesses said they saw people trapped inside the burning Grenfell Tower, in north Kensington, screaming for help, and shouting for their children to be saved.
More than 50 people are being treated in hospital, says London Ambulance.
The BBC's Andy Moore said the whole 24-storey block had been alight and there were fears the building might collapse.
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.
The Met Police has set up an emergency number on 0800 0961 233 for anyone concerned about friends or family.
London Fire Commissioner Dany Cotton said there had been "a number of fatalities" but she could not say how many because of the "size and complexity" of the building.
"This is an unprecedented incident," she told reporters."In my 29 years of being a firefighter, I have never ever seen anything of this scale."
She said the cause was not yet known.
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 Paramaseevan, 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."
The BBC's Andy Moore, who was at the scene, described watching debris falling from the building, and hearing explosions and breaking glass.
"The police keep pushing back their cordons, pushing back members of the public for fear the building might collapse," he said.
London Mayor Sadiq Khan said a "major incident" had been declared.
"Several hundred" people would have been in the block when the fire broke out, leader of Kensington and Chelsea Borough Nick Paget-Brown said.
Appeals are being made on social media for news of missing friends and relatives, who might have been caught in the blaze.
The first reports of fire in the tower, in Latimer Road, on the Lancaster West Estate, came in at 00:54 BST. Three hours later, people were still being evacuated from the tower, the police said.
Grenfell Tower was built in 1974 by Kensington and Chelsea Borough Council.
It's part of the Lancaster West Estate, a sprawling inner-city social housing complex of nearly 1,000 homes.
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 120 flats, is managed by the Kensington and Chelsea Tenant Management Organisation (KCTMO) on behalf of the council.
The local Grenfell Action Group had claimed, before and during the refurbishment, the block constituted a fire risk and residents had warned that access to the site for emergency vehicles was "severely restricted".
Council leader Nick Paget-Brown said the buildings were regularly inspected, but a "thorough investigation" was needed.
One eyewitness, George Clarke, the presenter of Channel 4 TV programme Amazing Spaces, told Radio 5 Live: "I'm getting covered in ash, that's how bad it is.
"I'm 100 metres away and I'm absolutely covered in ash.
"It's so heartbreaking, I've seen someone flashing their torches at the top level and they obviously can't get out."
Tim Downie, another eyewitness, told the BBC part of the building was "completely burned away".
"It has burned through to its very core," he said.
"It looks very bad, very very bad. I've never seen anything like this. It's just such a big fire.
"The whole building is just crumbling. It's just billowing black smoke."
Safiyah, who is about 500m away from the building, said: "There are lots of people gathered in the street. I just see more and more flames burning and tragically I hear people crying for help.
"The entire building is burning through."