Fire engulfed a 24-storey housing block in central London in the early hours on Wednesday, killing at least six people and injuring 74 others in an inferno that trapped residents as they slept.
Sadiq Khan has issued a statement, calling it a '"major incident" that is being tackled by 40 fire engines and 200 firefighters.
Some residents screamed for help from behind upper floor windows, some tried to throw children to safety, as flames raced through the high-rise Grenfell Tower block of apartments in the north Kensington area after taking hold just before 1 a.m.
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.
Nick Paget-Brown described the blaze as a “very, very severe fire”.
He told Sky News: “Clearly it's an absolutely devastating fire.
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.”
Another witness, Saimar Lleshi, saw people tying together sheets in an attempt to escape.
"I saw three people putting sheets together to climb down, but no one climbed down. I don’t know what happened to them."
"Even when the lights went off, people were waving with white shirts to be seen," Lleshi said.
More than 200 firefighters, backed up by 40 fire engines, fought for hours to try to bring the blaze, one of the biggest seen in central London in memory, under control.
By mid-day, London police said six people had been killed and they cautioned the death toll was likely to rise.