As many as 600 people are believed to have been inside Grenfell Tower's 120 flats when the blaze tore through the 24-storey building in the early hours of the morning.
Eighteen people are in critical care after 79 injured people were taken to hospital. But many are still missing after residents were left trapped on upper floors as flames rapidly ripped up the block after initially being told to stay in their homes.
Residents who escaped spoke of others trapped and screaming for help, with some throwing children from windows and others jumping from upper floors.
Some were reported to have attempted to use bin bags as makeshift parachutes.
Pictures showed flames engulfing the block and a plume of smoke visible across the capital, while others showed desperate residents looking out of windows in the block.
In a sign of hope, survivors were still being pulled from the block 12 hours after the blaze started - but numbers of those saved remains unclear.
London mayor Sadiq Khan said: "There will be a great many questions over the coming days as to the cause of this tragedy and I want to reassure Londoners that we will get all the answers."
More than 250 firefighters were called to the block in north Kensington at about 1am this morning. Several firefighters also suffered minor injuries in the blaze.
Commander Stuart Cundy, of the Metropolitan Police, said: "I can confirm twelve 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.
After fears were raised that the block could collapse, fire chiefs said a structural engineer is monitoring the stability of the building, which "continues to be safe for our crews to go and work in."