- Blast at Parsons Green station took place at 8.20 am local time
- Several people reported to have been injured
- Explosion reportedly took place inside a white bucket on train floor
"I was on the second carriage from the back. I just heard a kind of whoosh. I looked up and saw the whole carriage engulfed in flames making its way towards me," a man who was on the train told Reuters.
The explosion reportedly took place inside a white bucket in a supermarket freezer bag on the floor of one train's carriages.
A Metro.co.uk reporter at the scene was quoted by the paper as saying that a white container exploded on the train and passengers had suffered facial burns. She said they were "really badly burned" and their "hair was coming off".
Twitter user @Rrigs posted pictures of a white bucket smouldering on a train. Richard Aylmer-Hall, 52, a media technology consultant, described "panic" on the train. "There was panic, lots of people shouting, screaming, lots of screaming," he told the Press Association.
Mr Aylmer-Hall said he saw two women being treated by ambulance crews.
Outside the station, which has now been closed, a woman was sitting on a pavement with a bandage around her leg, while armed police patrolled. A witness reported seeing a woman being carried off on a stretcher with her legs covered in a foil blanket.
BBC correspondent Riz Lateef, who was at the station travelling to work, said there was "panic as people rushed from the train, hearing what appeared to be an explosion. "People were left with cuts and grazes from trying to flee the scene. There was lots of panic," he said.
"We are aware of an incident at #ParsonsGreen tube station. Officers are in attendance," London police said on Twitter, advising people to "avoid the area."
Police said they were aware of reports on social media and said they would release facts regarding the incident once they could be sure of their accuracy.
A Reuters witness could see a bomb disposal unit at the scene while the fire brigade said it had sent six engines and 50 firefighters. London Ambulance said it had sent "multiple resources" including its hazardous area response team to the scene. "Our initial priority is to assess the level and nature of injuries," it said.
Transport for London said on Twitter there was no service on the western part of the District Line which runs through Parsons Green.
Britain has suffered four attacks blamed on terrorists so far this year which killed 36 people. In 2005, 52 people were killed when four British Islamists carried out suicide bomb attacks on three London underground trains and a bus.