Emergency services attend the scene following a blast in an underground train at Parsons Green station in west London. Photograph: (Reuters)
At least 22 people were injured after a home-made bomb partially exploded on a packed commuter train heading into London, engulfing a carriage in flames.
"We now assess that this was a detonation of an improvised explosive device," Britain's top counter-terrorism officer Mark Rowley told reporters.
Rowley refused to disclose whether the suspected bomber had been on the train, saying it was a live investigation.
The police said this was Britain's fifth terrorism-related incident this year.
Some suffered burns while others were injured in the stampede which resulted as the passengers on board the train fled in panic as fire erupted at Parsons Green underground station in West London.
The incident took place at 8.20 am (0720 GMT).
"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," British Prime Minister Theresa May said.
"Everybody should keep calm and go about their lives in as normal a way as they possibly can," Foreign Secretary Boris Johnson told BBC TV.
Passengers were seen "badly burned", AFP reported.
A Metro.co.uk reporter at the scene was quoted by the paper as saying that some passengers were "really badly burned" and their "hair was coming off".
The police rushed to the spot.
Ambulances, fire engines were rushed to the Parsons Green station in West London.
Police were called at approx. 8:20am to #ParsonsGreen following reports of an incident on a tube train— Metropolitan Police (@metpoliceuk) September 15, 2017
An injured woman reacts outside Parsons Green tube station in London. (Reuters)