It was rush-hour and many commuters had entered an underground train in London when a blast was heard forcing many to scamper for safety.

Several of them were injured.

Police say it is  treating the incident as an act of terrorism.

No figure was given, but passengers on board a train heading into the capital fled as fire engulfed a carriage at Parsons Green underground station in West London at 8.20 a.m. (0720 GMT), with some suffering burns and other injuries in a stampede to escape, witnesses told Reuters.

The London ambulance service said it had taken 18 people to hospital, but said none were thought to be in a serious condition.

Neil Basu, the senior national coordinator for counter-terrorism policing, declared it a terrorist incident.

Prime Minister Theresa May will chair a meeting of Britain’s emergency response committee later on Friday, her office said.

Parsons Green underground train attack.

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May said in the statement on Friday: “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".

“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,” London police said in a statement.

Pictures taken at the scene showed a white bucket with a supermarket freezer bag on the floor of one train carriage. The bucket was in flames and there appeared to be wires coming out of the top.

“I was on second carriage from the back. I just heard a kind of whoosh. I looked up and saw the whole carriage engulfed in flames making its way toward me,” Ola Fayankinnu, who was on the train, told Reuters.

“A lot of people were trampled on. There were phones, hats, bags all over the place and when I looked back I saw a bag with flames. People were crying, shocked, a few people had been injured, some people had been trampled.”

Foreign Secretary Boris Johnson, has also asked people should “keep calm” and continue their lives as normal.