It’s not yet clear what caused the security alert, which occurred at about 8:20 a.m. local time (3:20 a.m. ET) at the height of rush hour on a busy commuter line into central London.
But witnesses told CNN they heard announcements from officials that “some kind of explosion” had occurred on a train at Parsons Green Tube station.
Deputy Assistant Commissioner Neil Basu, the senior national coordinator for counterterroism policing, has declared it a terrorist incident, the Met Police said in a statement.
Parsons Green station is closed, a large security cordon has been put in place and large numbers of emergency responders are at the scene.
London’s Metropolitan Police earlier tweeted: “We are aware of an incident at Parsons Green tube station. Officers are in attendance.”
London Ambulance Service said a number of responders, including its hazardous area response team, were at the scene alongside the police.
Natasha Wills, the ambulance service’s assistant director of operations, said: “Our initial priority is to assess the level and nature of injuries. More information will follow when we have it.”
Software developer Sylvain Pennec told the UK’s Press Association news agency he heard a “boom and when I looked there were flames all around.”
“People started to run but we were lucky to be stopping at Parsons Green as the door started to open.”
Richard Aylmer-Hall, 52, a media technology consultant, described “panic” on the train in an interview with PA .
“There was panic, lots of people shouting, screaming, lots of screaming,” he said.
“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.”
The Met Police said it was “aware of reports on social media” about Parsons Green, adding: “We will release facts when we can — our info must be accurate.”
Large numbers of emergency responders, including armed police officers, could be seen near Parsons Green station.
The security cordon around the station, which is above ground, has been extended and emergency vehicles could be seen continuing to arrive over an hour after the incident.
Parsons Green is an affluent, mainly residential area popular with young families in the borough of Hammersmith and Fulham, not far from Chelsea.
Train services between Earl’s Court and Wimbledon on the District Line have been suspended, Transport for London said. Customers are advised to take alternative routes.
Developing story – more to come
CNN’s Simon Cullen and Lauren Said-Moorhouse contributed to this report.