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Neil Basu, Metropolitan Police Deputy Assistant Commissioner and the senior national coordinator for counter-terrorism policing, said it was a terrorist incident, the Guardian reported.
Twenty two people were hospitalised after the blast at 8.20 a.m. at the Parsons Green station on District Line in southwest London.
Assistant Commissioner Mark Rowley said a suspected Improvised Explosive Device (IED) was thought to be responsible for the blast. A timer was attached to the explosive device, reports said.
The London Ambulance Service said none of the injured were believed to be in serious condition.
Downing Street announced that Prime Minister Theresa May would be presiding over a Cobra emergency committee meeting to discuss the incident. The country's threat level currently stands at "severe", the second-highest behind "critical".
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 rushed 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". He said a "manhunt was underway" following the explosion. "The person or people responsible for this attack will be caught," he said.
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.