At least six people have died after a huge fire destroyed a tower block in west London, but the death toll is expected to rise.
Flames tore up the 24-storey Grenfell Tower in north Kensington, leaving people trapped on upper floors, including children, some holding babies from windows and others jumping from their flats.
Commander Stuart Cundy, of the Metropolitan Police, said: "I can confirm six fatalities at this time but this figure is likely to rise during what will be a complex recovery operation over a number of days. Many others are receiving medical care."
He said it was likely to be some time before police are able to identify the victims, adding that it was too early to speculate on the cause of the fire.
People who escaped the fire 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.
Witnesses have told how they were woken by screaming in the early hours after a fire hit 27-storey Grenfell Tower in north Kensington.
Joanna O'Connor, a local resident, told Sky News: "At about 2am woken up by screaming, sirens and helicopters. We came outside and were confronted with the building that was completely engulfed in flames.
"It was a real shock and there were hundreds of people lining the streets, we could still hear screaming from the building and people were milling around in shock crying.
"One of our neighbours, her sister, husband and children were in the building, it was their neighbours' flat that caught fire. So it's very close to us, we've got neighbours whose families are in that building."
More than 200 firefighters were sent to tackle the fire.
Police said a "number of people are being treated for a range of injuries", including two for smoke inhalation, as pictures from the scene in north Kensington showed flames engulfing the block and the plume of smoke visible across the capital.
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 onto 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.
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.
T he cause of the fire is 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.
Stuart Crighton, London Ambulance Service assistant director of operations, said: "We have sent a number of resources to the scene including our hazardous area response team and over 20 ambulance crews.
"Our priority (is) to get people to safety and ensure they receive the medical help as quickly as possible.
"Our initial priority is to assess the level and nature of injuries and ensure those in the most need are treated first and taken to hospital."
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.
Kensington and Chelsea Council leader 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."
London Fire Brigade said around 30 flats near the scene had been evacuated and a cordon was in place.
An acrid column of smoke could be seen rising from the building shortly before 7am.
The charred structure still had pockets of flame rising from several storeys as the desperate effort 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.