LONDON • Witnesses heard a loud bang and saw a "wall of fire" when an improvised bomb went off on a packed London Underground train. As flames engulfed one carriage and raced along the train on a London route to Parsons Green, they said passengers panicked and trampled on one another in a rush to an exit.

Mr Charlie Craven was heading to the Parsons Green station on his way to work in the city when he heard a "massive bang" as the device detonated. "The first thing I saw was an orange sort of fireball encompassing" the whole Tube train. "Bit like you see on 24, the TV show, or a movie sort of situation."

Mr Adam Davis, a 23-year-old student, said he was in the train car where the device exploded.

"I had my headphones on, then I felt a kind of vibration, followed by a wave of heat, and I looked down and the whole carriage was in flames," he said. "I just got up and ran, but the carriage door was jammed with people. Everyone was screaming and trying to get out. People had blood on them, everyone was pushing. It was like a stampede."

Mr Davis added: "I just saw flames and you think the worst. You think bomb. Terrorism."

Another passenger, Mr Ola Fayankinnu, told a similar story. "I just heard a kind of whoosh. I looked up and saw the whole carriage engulfed in flames making its way towards me," he said.

Outside the station, a woman sat on a pavement with a bandage on her leg, while armed police patrolled. A witness saw a woman carried off on a stretcher with her legs in a foil blanket.

Ms Natalie Belford, 42, a hairdresser and beautician, said: "The train was packed ... I heard a big boom and felt this heat on my face. I ran for my life, but there was no way out. The doors were full of people and the carriage was too packed."

She said she was knocked over twice, and showed a reporter her ripped tights and bloodied knees.

"I knew it was a bomb when I saw people with charred hair and burned faces," Ms Belford said. "This has got to be terrorism - a bag full of explosive materials doesn't just appear on a train by accident."

Alex Ojeda-Sierra, 13, a pupil at the London Oratory School, was on his way to school when heard screaming from other carriages.

"I was talking with my friend and all of the sudden people started screaming and running," he said in an interview at Chelsea and Westminster Hospital, displaying several bruises on his face. "One man fell on me and I had my legs bent backwards and my right ankle got twisted and I started screaming that I had no air."

REUTERS, NYTIMES, AGENCE FRANCE-PRESSE