Eighteen people were injured at a London underground station on Friday after witnesses reported a blast on a packed rush-hour commuter train, Reuters reported.

“It is too early to confirm the cause of the fire, which will be subject to the investigation that is now underway by the Metropolitan Police’s Counter Terrorism Command,” London police said in a statement.

Commissioner Neil Basu, the senior national coordinator for counter-terrorism policing, declared the blast a terrorist incident, the statement said.

Train services between Earl’s Court and Wimbledon on the District Line were suspended, Transport for London said. London Ambulance Service said a number of responders, including its hazardous area response team, were at the scene alongside the police, CNN reports.

Natasha Wills, the ambulance service’s assistant director of operations, told CNN that “Our initial priority is to assess the level and nature of injuries. More information will follow when we have it.” She confirmed in a statement that 18 people were taken to hospitals.

An unverified picture circulating on social media showed a white bucket with a supermarket freezer bag on the floor of one train carriage, Reuters reported. The bucket was in flames and there appeared to be wires coming out of the top.

A witness told Reuters that there were six ambulances, fire engines and a helicopter at the Parsons Green station. The Metro newspaper reported that passengers had suffered facial burns from a blast and others had been hurt in a subsequent stampede, according to Reuters.

“I was on (the) 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 towards 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.”

Outside the station, a woman was sitting on a pavement with a bandage around her leg while armed police patrolled, Reuters reported. A witness saw a woman being carried off on a stretcher with her legs covered in a foil blanket, Reuters reported.

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