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 at Grenfell Tower, in Latimer Road.
He told the BBC: "I grabbed an axe from the fire truck, it looked like there was a bit of confusion about what to do.
"I ran around the building looking for a fire escape and couldn't see any noticeable fire escapes around the building. A lot of debris falling down.
"I eventually gained entry on to the second floor, and once I got to the corridor I realised there was so much smoke there."
He added that given the thickness of the smoke, he would be surprised if anyone could have left the building without assistance.
"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'," he said.
Nick Paget-Brown described the blaze as a "very, very severe fire".
He told Sky News: "Clearly it's an absolutely devastating fire.
"Several hundred would have been in there. It's a question of establishing how many people were in there at the time of the fire.
"I'm really not in any position to answer any questions about the structure.
"Clearly there's a lot more work to do to evacuate the building and to establish how safe it is."
Fire crews from north Kensington, Kensington, Hammersmith and Paddington and surrounding stations were at the scene with the fire burning from the second to the top floor.
The cause of the fire was not known at this stage, London Fire Brigade said.
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."
London Fire Brigade assistant commissioner Dan Daly said: "Firefighters wearing breathing apparatus are working extremely hard in very difficult conditions to tackle this fire.
"This is a large and very serious incident and we have deployed numerous resources and specialist appliances."
London Ambulance said it had sent a " number of resources" to the scene, including its Hazardous Area Response Team.
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."
Transport for London said there was no service between Hammersmith and Edgware Road on the Circle and Hammersmith and City lines, while the police said the A40 was closed in both directions, owing to the fire.
London Fire Brigade said around 30 flats near the scene had been evacuated and a cordon was in place.