Commuters were reportedly injured in an explosion at Parsons Green underground station in west London.
Transport for London (TfL) said the District Line was partially suspended because of a “security alert”.
TfL 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.”
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”.
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.”
Social media reports claimed a bag had exploded, with pictures showing smouldering debris on a District line train. One apparent witness claimed: “Fireball flew down carriage and we just jumped out open door.”
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.”