Commuters were reportedly injured following an explosion at Parsons Green underground station in west London.
Witnesses described seeing people with burns and, separately, injuries caused when passengers rushed to escape the station.
Transport for London (TfL) said the District Line was partially suspended because of a “security alert” at about 8.20am.
The authority referred The Independent to the British Transport Police (BTP). A BTP spokesman said: “We’re aware of an incident at Parsons Green station. Officers are at the scene. The station is closed.” Counter-terror and security authorities are monitoring the situation.
The Metropolitan Police said they were aware of the incident but could not provide further information. Armed police were reportedly at the station.
Firefighters and paramedics have been called, and the Metropolitan Police said it was aware of the incident and its officers were also on scene.
Witnesses told of an “extremely heavy police presence” and “quite a few people injured”, potentially including facial burns. A number of injuries appeared to have stemmed from a crush of people trying to leave the station.
Social media reports claimed a bag had exploded, with pictures showing smoldering debris on a District line train. One apparent witness claimed: “Fireball flew down the carriage and we just jumped out the open door.”
Images showed a white bucket—seemingly with wires trailing from it—inside what appeared to be a Lidl supermarket freezer bag.
BBC News presenter Sophie Raworth said she saw a woman on a stretcher with burns to her face and legs, the broadcaster reported.
Media technology consultant Richard Aylmer-Hall, 53, was sitting on the District Line train bound for central London when panic unfolded at about 8.20am. Several people were injured after passengers tried to escape, he said.
He added: “Suddenly there was panic, lots of people shouting, screaming, lots of screaming.
“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.
“Some people got pushed over and trampled on, I saw two women being treated by ambulance crews.”
Daniel Holden, a Merton borough councillor, told the London Evening Standard he experienced “a loud bang, heatwave and fire burst”.
Aerial images showed passengers being taken off the train.
A London Fire Brigade spokeswoman said: “The brigade was called at 8.21am. Fire crews from Fulham, Wandsworth, Chelsea, Hammersmith and other surrounding fire stations are in attendance.”
Six fire engines and about 50 firefighters, plus specialist officers, were sent to the station, she said.
A London Ambulance Service spokeswoman said: “We have sent multiple resources to the scene including single responders in cars, ambulance crews, incident response officers and our hazardous area response team, with the first of our medics arriving in under five minutes.”
Tube services between Wimbledon and Edgware Road are suspended.