Parents wrapped wet towels around their children as they desperately tried to escape from the Grenfell Tower, with at least one person seen waving a piece of white cloth from an upper floor as flames ravaged the building.
"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," said police commander Stuart Cundy, describing the fire as "truly shocking".
The ambulance service said 50 people had been hospitalised.
The alarm was raised just before 1:00 am (0000 GMT) and within an hour flames had engulfed the entire block.
Residents claimed the fire was spreading on the exterior of the tower, which had been covered in cladding in a major refurbishment completed last year.
More than 10 hours on, flames could still be seen inside the charred building as thick, black smoke filled the sky.
London Mayor Sadiq Khan said firefighters were only able to reach the 12th floor at the height of the blaze.
"A lot of people are unaccounted for," he told Sky News television.
Nick Paget-Brown, leader of the Kensington and Chelsea local authority, said "several hundred" people would have been inside the block.
London Fire Brigade chief Dany Cotton said the building's structure was stable enough for fire crews to work inside.
"We rescued a large number of people from inside the building very early on," she said, adding firefighters had managed to reach the 19th and 20th floors.
"We are making steady progress."
Large pieces of debris could be seen falling from the building, a 1970s local authority-built block in the working-class north Kensington area.
