Update (15 September 5pm GMT): A total of 22 people have been treated in hospitals following the incident, mostly with burns, and at least eight have been discharged, the BBC reported. Police are looking for the person who planted the "improvised explosive device" and the area around Parsons Green station has been evacuated.
Theresa May said the UK's terror threat level would remain at the second highest level – severe – but would be reviewed.
Update (15 September 11am GMT): The Metropolitan Police have confirm that the incident is being treated as terrorism, reported the BBC. Prime Minister Theresa May tweeted: "My thoughts are with those injured at Parsons Green and emergency services who are responding bravely to this terrorist incident."
Original story: Multiple people are said to be injured after an explosion on a District Line train at Parsons Green station in southwest London.
Photos and videos from the scene show a white bucket on fire inside a plastic supermarket bag. The inside of the Tube train carriage does not appear to be damaged, but witnesses have reported seeing people with injuries, including burns to the face, arms and legs, reported The Guardian.
Police and paramedics were called to the scene at 08:20 BST (07:20 GMT), the BBC reported. Natasha Wills, assistant director of operations at London Ambulance Service, said "multiple resources" had been sent 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," reported the BBC.
"Our initial priority is to assess the level and nature of injuries. More information will follow when we have it."
Passengers have reported "panic" at the scene, with BBC London presenter Riz Lateef, who was on her way to work, saying: "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."
Sophie Raworth, another BBC News presenter, said she saw a woman with multiple burns being carried from the scene on a stretcher.
The British Transport Police called on people to remain vigilant and report suspicious behaviour.
London Mayor Sadiq Khan said he was in touch with the Metropolitan Police about the incident and urged the public to follow the force's Twitter account for updates.
This is a breaking news story. We will update it as more details emerge.