Several people have died after a huge fire destroyed a tower block in west London with witnesses reporting residents caught in the flames.
People who escaped the fire at the 27-storey Grenfell Tower in north Kensington spoke of others trapped and screaming for help, with some holding children from windows and others jumping from upper floors.
London Fire Commissioner Dany Cotton said there had been a "number of fatalities" but could not say how many due to the size and complexity of the building.
She told reporters at the scene: "This is an unprecedented incident. In my 29 years of being a firefighter, I have never ever seen anything of this scale."
Pictures from the scene showed flames engulfing the block and a plume of smoke visible across the capital, while others showed residents looking out of windows in the block.
The leader of Kensington and Chelsea Borough Nick Paget-Brown said "several hundred" people would have been in the block when the fire broke out.
Actor and writer Tim Downie, who lives around 600 metres from the scene in Latimer Road, told the Press Association he feared the block could collapse.
He said: "It's horrendous. The whole building is engulfed in flames. It's gone. It's just a matter of time before this building collapses.
"It's the most terrifying thing I've ever seen. I just hope they have got everyone out.
"The first I knew was the noise of sirens, helicopters and shouting. I saw it engulfed in flames.
"People have been bringing water, clothes, anything they've got to help, out to the cordon.
"I have seen people coming out in their bedclothes - it's just very distressing."
Jody Martin said he got to the scene just as the first fire engine was arriving.
He told the BBC: "I watched one person falling out, I watched another woman holding her baby out the window... hearing screams, I was yelling everyone to get down and they were saying 'We can't leave our apartments, the smoke is too bad on the corridors'."
Fabio Bebber wrote on Twitter: "More screams for help as the fire spreads to another side of the building.
"We can see how quick the fire spreads via the external panels. It's unbearable hearing someone screaming for their lives at #grenfelltower."
George Clarke, who presents the Channel 4 TV show Amazing Spaces, told Radio 5 Live: "I was in bed and heard 'beep, beep, beep' and thought, 'I'll get up and run downstairs as quickly as I could'.
"I thought it might be a car alarm outside and saw the glow through the windows.
"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.
"The guys are doing an incredible job to try and get people out that building, but it's truly awful."
Shortly after 6am, London Ambulance Service said 30 people had been taken to five hospitals.
More than 200 firefighters were sent to tackle the blaze which was reported just before 1am on Wednesday.
Firefighters were on the scene within six minutes.
London mayor Sadiq Khan tweeted: "Major incident declared at Grenfell Tower in Kensington" and urged people to follow London Fire Brigade on Twitter.
Former chancellor and now editor of the Evening Standard George Osborne tweeted: "Just seen this awful tower block fire near my home in W London. My prayers with those affected & heroes tackling it."
The cause of the fire was not known at this stage, London Fire Brigade said.
But residents said refurbishment work had recently been carried out with cladding on the outside of the structure and work on the gas supply to the flats.
A residents action group said their warnings about safety had fallen on "deaf ears".
A blog post from the Grenfell Action Group from November 2016 said "only a catastrophic event" would expose the concerns residents had.
The group said there was one entry and exit to Grenfell Tower during improvement works at the block and it had issues with evacuation procedures.
Following the fire, the group posted: "All our warnings fell on deaf ears and we predicted that a catastrophe like this was inevitable and just a matter of time."
Nicky Paramasivan, who was in his seventh floor flat with his partner and child, said the advice issued to residents in the event of a fire was to stay in their flats.
"If we'd listened to them and stayed in the flat we'd have perished," he told the BBC.
He said that after they fled the explosions from the flats included blue flames, suggesting gas.
"It's just horrendous. I'm just hoping there's not too many dead.
"It went right through. There weren't no stopping it."
An acrid column of smoke could be seen rising from the building on Wednesday.
The charred structure still had pockets of flame rising from several storeys as desperate efforts to bring the blaze under control continued.
Schoolboy Omar Kalam, 11, was standing anxiously at the emergency service cordon with father Walid, 44.
"My brother has friends and they live in there," he said. "I'm not sure if they are all right yet."
Parents from nearby Kensington Aldridge Academy, where Omar attends, had been told the school was closed, his father said.
The Metropolitan Police have set up a casualty bureau for anyone concerned about their friends and family on 0800 0961 233.