It was not clear if residents were trapped, but people at the scene spoke of being unable to reach friends and family who had been inside. The cause of the fire was not immediately known. Residents said it appeared to start in an apartment on a lower floor and spread upward quickly.
London Mayor Sadiq Khan said on Twitter that a major incident had been declared.
The blaze started around 1 a.m. London time, and smoke was still pouring from the building more than six hours later.
Nassima Boutrig, who lives opposite the building, said she was awakened by sirens and smoke so thick that it filled her home as well.
“We saw the people screaming,” she said. “A lot of people said ‘Help, help, help.’ The fire brigade could only help downstairs. It was fire up, up, up. They couldn’t stop the fire.”
Boutrig said her friend’s brother, wife and children lived in the building and that her friend was waiting to find out if they were OK.
Ambulances and firetrucks filled the streets around the building, which is located in a diverse, working class area of London. People who live nearby were evacuated, some carrying pets in their arms as they left. Volunteers handed out bottled water.
Helicopters hovered overhead and smoke hung over the scene. Exhausted firefighters sprawled on the pavement just inside the police cordon, drinking water from plastic bottles.
The London Fire Brigade said 45 fire engines and 200 firefighters were called to the scene. Assistant Fire Commissioner Dan Daly said it was a large and very serious incident.
“Firefighters wearing breathing apparatus are working extremely hard in very difficult conditions to tackle this fire,” he said in a post on the brigade’s Facebook page.
George Clarke told Radio 5 Live that he was covered in ash even though he was 100 meters (yards) from the scene.
He said he saw people waving flashlights from the top levels of the building and saw rescuers “doing an incredible job” trying to get people out.
Tim Downie, who lives not far away, told Britain’s Press Association that he feared the building could collapse. He said he heard sirens, helicopters and shouting and then saw the building engulfed in flames.
“It’s the most terrifying thing I’ve ever seen. I just hope they have got everyone out,” he said. “People have been bringing water, clothes, anything they’ve got to help, out to the cordon.
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