ALSO READBlast at Parsons Green tube station in London, several injured: Report London attack: Why terror threat level downgraded to severe from critical London attackers strapped fake explosives for 'maximum fear' 8 minutes on London Bridge: Years of training led to prompt police response Terror visits the UK again: 7 killed in London attack, 3 suspects shot dead
The incident occurred during rush hour on a District Line Tube train as it pulled into Parsons Green station.
Several people are said to have suffered facial burns after the blast in a bucket towards the rear of the District Line train. Some commuters reportedly were trampled and suffered crush injuries in the rush to escape.
Scotland Yard's counter-terrorism squad, SO15, arrived at the scene at Parsons Green Tube station and took the lead in the investigation from the British Transport Police.
"Deputy Assistant Commissioner Neil Basu, the senior national coordinator for counter-terrorism policing, has declared it a terrorist incident," the Metropolitan Police said in a statement.
"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," the statement said.
"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," it said.
Downing Street said Prime Minister Theresa May was "receiving regular updates" on the situation.
Some of the injuries are believed to be related to the direct impact of the explosion involving a large white plastic bucket in a supermarket plastic bag in one of the carriages. Images of the bucket with wires sticking out have emerged on social media.
However, many commuters were also injured in the chaos that followed as people panicked and tried to escape from the scene, resulting in a stampede at the station.
While the exact nature of the explosion is not yet known, Metropolitan Police bomb squads and fire crews remained at the scene including London Ambulance's Hazardous Response Units.
"Our initial priority is to assess the level and nature of injuries," it said.
Eyewitnesses from shops outside the station had reported seeing people with facial injuries. Many have been told to stay in lockdown as police evacuated the area and placed a wide zone under a police cordon.
Passenger Chris Wildish told the BBC that he saw a bucket in a supermarket bag with "low-level flames coming out of it" by the door of the rear carriage.
(Only the headline and picture of this report may have been reworked by the Business Standard staff; the rest of the content is auto-generated from a syndicated feed.)