Terrified passengers fled Parsons Green station following reports of an explosion on board a packed District Line train at around 8.20am on September 15.
Commuter Peter Crowley was caught in the explosion and tweeted images of his singed scalp and hair at around 10am.
"A fireball above my head has come and singed all my hair," he told BBC News.
"I've got burn marks on the top of my head and everyone just ran off the train - it was quite scary."
Other passengers are said to have "suffered facial burns."
London Ambulance Service confirmed 18 people were injured, with non suffering life-threatening injuries.
A security alert was raised at 8.20am following reports of an explosion, triggering panic among commuters during rush hour.
Police are now treating the event as a terror incident.
Assistant Commissioner Mark Rowley from the Met Police has said the attack at Parsons Green was the result of an improvised explosive device (IED).
He said: "We now assess that this was a detonation of an improvised explosive device.
"The scene currently remains cordoned off and the investigation continues.
"In terms of the police response, our well-practised command structures have come into shape.
"There are many urgent inquiries ongoing with hundreds of detectives involved looking at CCTV, forensic work and speaking to witnesses."
The Wimbledon District Line service was suspended and the Metropolitan Police and emergency services attended the incident.
Large crowds gathered outside Parsons Green station as passengers fled from the incident.
Media technology consultant Richard Aylmer-Hall, 53, was sitting on the District Line train bound for central London when panic unfolded.
He said he saw several people injured, having apparently been trampled as they tried to escape.
He said: "I was on my way up from Wimbledon towards Paddington and suddenly there was panic, lots of people shouting, screaming, lots of screaming.
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