Neil Basu, Metropolitan Police Deputy Assistant Commissioner and the senior national coordinator for counter-terrorism policing, said some hours after the incident that it was a terrorist incident, reports the Guardian.
Eighteen people have been hospitalized after they were injured in the blast at 8.20 a.m. at the Parsons Green station on District Line in south-west London.
Assistant Commissioner Mark Rowley says a suspected Improvised Explosive Device (IED) was thought to be responsible for the blast.
Passengers reported seeing a device on the train and hearing a bang during the morning rush hour.
Witnesses described seeing at least one passenger with facial injuries. Others spoke of panic as stunned passengers trooped out of the train.
Pictures on social media showed a white bucket on fire inside a supermarket bag but did not appear to show extensive damage to the inside of the train carriage, the BBC reported.
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."
London Mayor Sadiq Khan appealed for calm, saying the city "will never be intimidated or defeated by terrorism".
Train services between Earl's Court and Wimbledon on the District Line were immediately suspended.
Meanwhile, US President Donald Trump urged the British police to be "pro-active" in tackling terrorism.
"Another attack in London by a loser terrorist. These are sick and demented people who were in the sights of Scotland Yard. Must be pro-active.
"Loser terrorists must be dealt with in a much tougher manner. The internet is their main recruitment tool which we must cut off and use better," he said in a series of tweets.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)