A total of 22 people - including children - were injured in the explosion, with majority suffering from "flash burns and are being treated in hospital, mostly for burn injuries".
Flames engulfed one carriage and raced along a train on a west London route to Parsons Green, forcing passengers to trample others as they rushed for an exit, a witness told Reuters news agency. As a manhunt takes place across the capital, authorities have tightened security on all transport hubs, The New York Times reports.
An underground train in London was hit with a detonated device in an apparent terrorist attack.
The station has been cordoned off and train services have been suspended after the incident, which happened during rush hour.
The area surrounding the station has been evacuated while emergency services continue to work at the scene of the incident.
United Kingdom police and London ambulance services are responding to the incident at Parsons Green station, southwest of the capital.
Pictures and video posted on social media show a white plastic bucket inside a foil-lined shopping bag, which appeared to have wires coming from it, on the floor of the subway vehicle.More news: Six New 360 Titles Join Xbox One Backwards Compatibility Today
The official threat level now stands at severe - meaning an attack is seen as "highly likely".
The Metropolitan Police said none of the injured sustained life-threatening or serious injuries.
Friday's fire was the fifth terrorism-related attack in London in 2017.
May said in a televised statement that armed police and members of the military would be seen on the streets in the coming days.
Police in London are treating an explosion on a packed commuter train Friday as a terror attack.
British Prime Minister Theresa May said her thoughts were with the injured.