London Fire Brigade chief Dany Cotton said there had been “a number of fatalities” and 30 people had been hospitalised after what she called an “unprecedented” blaze.
Eyewitnesses said they had seen some people fall or jump from the stricken building and at least one resident waving a piece of white cloth from one of the upper floors as flames ravaged the tower.
Seven hours after the alarm was raised, flames could still be seen inside the charred building as a thick plume of black smoke marred the west London skyline.
Large pieces of debris could be seen falling from Grenfell Tower, a 1970s council block in the working-class north Kensington area — a short distance from chic Notting Hill.
Witnesses said they heard screaming from the upper floors as the flames rose in the night.
“They were trapped. They couldn’t come downstairs, especially from the top floor … people have been burned,” a witness identified as Daniel told BBC Radio London.
“I have seen it with my own eyes. And I have seen people jump.”
‘I just ran’
Hanan Wahabi, 39, told AFP she escaped with her husband and son, 16, and daughter, 8, but feared for her brother and his family who live on the 21st floor.
“Last time I saw him they were waving out the window, his wife and children,” she said, sitting outside the Rugby Portobello community centre.
“I’ve not heard from them since, the phone is not going through, the landline isn’t going through. That was about 2:00 am (0100 GMT), she said.
Eddie, 55, who lives on the 16th floor, said he ran out of his house with a wet towel wrapped around his head as smoke engulfed the flat.
“You couldn’t see anything. I just ran down the stairs… Loads of people haven’t got out of the building,” he said.
Another witness named as Jody Martin told the BBC that he battled his way his way to the second floor only to encounter choking smoke.
“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.
By early morning, most of the block was a smouldering hulk as firefighters sprayed water onto floors within reach of appliances on the ground.
Frantic families at the scene attempted to call their loved ones, fearing they could be stuck inside, and were being directed by police to a nearby restaurant where some of the injured were being treated.
The fire brigade said 40 fire engines and 200 firefighters had been called to the blaze at Grenfell Tower, which has 120 flats.
“Fire is from second to top floor of 27 storey building,” the fire service said on Twitter.
Firefighters said they had managed to evacuate residents up to the 11th floor.
“We can confirm that we have taken 30 patients to five London hospitals following the incident,” said Stuart Crichton, assistant director of operations at the London Ambulance Service.
He added that more than 20 ambulance crews as well as a “hazardous area response team” were at the scene.
“Our priority is to assess the level and nature of injures and ensure those in the most need are treated first and taken to hospital.”
Police cleared nearby buildings because of fears about falling debris and shut down a section of the A40 highway — a normally busy thoroughfare into London.
A London Underground line passing the area near Latimer Road station was also shut down.
“We’ve set up emergency centres nearby and people have been evacuated to those,” local councillor Nick Paget-Brown said on Sky News.
“It’s clearly been a devastating fire,” he said. “There’s a lot more work to do to evacuate the building and to establish how safe it is.”
The apartment block was built in 1974, but had recently undergone a major refurbishment, including a new heating and hot water system.
The refurbishment cost £8.7 million (9.9 million euros, $11 million) and was completed in May 2016.
The exterior was modernised with cladding.
Local residents had warned a year ago about a potential fire risk caused by rubbish being allowed to accumulate during improvement works.
“This matter is of particular concern as there is only one entry and exit to Grenfell Tower during the improvement works,” read a blog post by the Grenfell Action Group.
“The potential for a fire to break out in the communal area on the walkway does not bear thinking about as residents would be trapped in the building with no way out,” it said.