London - A massive fire raced through a high-rise apartment building in west London early on Wednesday, sending at least 30 people to hospitals, emergency officials said.Flames shot from windows all the way up the side of the 24-storey Grenfell Tower in North Kensington as firefighters battled the blaze, and a plume of smoke could be seen for several kilometres. London Mayor Sadiq Khan said on Twitter that a major incident had been declared.The blaze started around 01:00 London time, and ambulance crews were still at the scene more than five hours later. It was not clear if residents were trapped, but people at the scene spoke of not being able to reach friends and family who had been inside.Nassima Boutrig, who lives opposite the building, said she was awakened by sirens and smoke so thick that it filled her home as well.'Help, help, help'"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.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, the presenter of Amazing Spaces, told Radio 5 Live that he was covered in ash even though he was 100m 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."