Emergency services said they were responding to an "incident" in which people were injured at an Underground station in west London on Friday, following media reports of an explosion.
"We are aware of an incident at Parsons Green tube station. Officers are in attendance," London's Metropolitan Police said on Twitter.
The station was closed, as well as an entire section of the District Line where it is located.
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.
"There was a woman on the platform who said she had seen a bag, a flash and a bang, so obviously something had gone off," he said.
"I saw two women being treated by ambulance crews," he said.
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."
Natasha Wills, assistant director of operations at London Ambulance Service said in a statement: "Our initial priority is to assess the level and nature of injuries".
She said the ambulance service had sent "multiple resources" to the station, including a hazardous area response team.
London Fire Brigade said they were also at the scene and were called at 8:21 am.