London police have confirmed they are treating the explosion which occurred at the Parsons Green Tube station as a terrorist incident.
At least 18 people have been injured by the explosion and subsequent stampede on the District Line train in West London, which occurred at around 8:20am on 15 September.
Pictures taken from the scene appear to show a white bucket inside a plastic bag which is believed to have been the source of the explosion on the busy rush hour train.
An eyewitness from the scene described how a "fireball flew down [the] carriage" following the explosion, causing mass panic and what was described as a "stampede" out of the train.
The London Ambulance Service confirmed they have sent "multiple resources" including their hazardous area response team to the scene to deal with the incident.
They confirmed 18 patients were taken to a number of London hospitals, none of who are thought to be in a serious or life-threatening condition.
Police are warning people to stay away from the area while they investigate the incident, which is being dealt with by the Met's Counter Terrorism Command.
Downing Street has confirmed that Prime Minister Theresa May is to chair a meeting of the government's Cobra emergency committee later this afternoon to discuss the suspected terrorist incident.
May said: "My thoughts are with those injured at Parsons Green and the emergency services who, once again, are responding swiftly and bravely to a suspected terrorist incident."
In response, Mayor of London Sadiq Khan that the capital has proven "time and time again" that it will "never be intimidated or defeated by terrorism. The Metropolitan Police have confirmed that the explosion on a train at Parsons Green Station this morning is being treated as terrorism.
"Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life. I am in close contact with the Metropolitan Police, Transport for London, Government and other emergency services who are responding at the scene and leading the investigation. I will be attending the emergency COBRA meeting in Whitehall this afternoon with the Prime Minister.
"My sincere gratitude goes to all our courageous emergency responders and the TfL staff who were first on the scene. I urge all Londoners to remain calm and vigilant, and to check TfL's website for travel advice."
A Met Police spokesperson said: "Officers from the Met's Counter Terrorism Command are investigating after an incident on a London Tube train this morning. Police were called at approximately 08:20hrs on Friday, 15 September, to Parsons Green Underground Station following reports of a fire on the train.
"Deputy Assistant Commissioner Neil Basu, the senior national coordinator for CT policing, has declared it a terrorist incident. Officers from the Metropolitan Police Service and British Transport Police attended the scene, along with colleagues from the London Fire Brigade and London Ambulance Service.
"At present we are aware of a number of people who have suffered injuries. It is too early to confirm the cause of the fire, which will be subject to the investigation that is now underway by the Met's Counter Terrorism Command. The station remains cordoned off and we are advising people to avoid the area."