Eyewitnesses said they saw people trapped inside the burning Grenfell Tower, in north Kensington, screaming for help, and shouting for their children to be saved.
Firefighters, still at the scene, rescued "large numbers" of residents.
The 24-storey block, now smoking ruins, looks at risk of collapsing.
Eyewitnesses said they saw lights - thought to be mobile phones or torches - flashing at the top of the block of flats, and trapped residents coming to their windows - some holding children.
The Met Police has set up an emergency number on 0800 0961 233 for anyone concerned about friends or family.