An improvised explosive device (IED) on Friday went off on an underground train during the morning rush hour at a busy station in London, leaving at least 22 people injured, in what Scotland Yard called a “terrorist incident”.

The explosion happened on a District Line Tube as the train pulled into Parsons Green station in southwest London.

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Scotland Yard said a suspected “bucket bomb” exploded and the incident was being treated as a “terrorist attack”.

The London Ambulance Service said 18 people have been taken to hospital, and four more reached there on their own.

Most of the injured people suffered burn injuries.

Assistant Commissioner Mark Rowley said they “assess” an IED triggered the blast. He said more police would be deployed in London, particularly on the transport network but refused to say whether anyone had been arrested.

British media reported of pictures of a white bucket on fire inside a supermarket bag, with wires trailing on to the carriage floor of the train.

Several people were said to have suffered facial burns.

Some reportedly were trampled in the rush to escape.

Scotland Yard’s counter-terrorism squad, SO15, arrived at the scene at Parsons Green Tube station and took the lead in the investigation from the British Transport Police.

“Deputy Assistant Commissioner Neil Basu, the senior national coordinator for counter-terrorism policing, has declared it a terrorist incident,” the police said.

“The Metropolitan Police Service and British Transport Police attended the scene, along with colleagues from the London Fire Brigade and London Ambulance Service. At present we are aware of a number of people who have suffered injuries,” the police said in a statement.

“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. The station remains cordoned off and we are advising people to avoid the area,” it said.

Downing Street said British Prime Minister Theresa May was “receiving regular updates” on the situation.

“My thoughts are with those injured at Parsons Green and emergency services who are responding bravely to this terrorist incident,” May tweeted.

The police bomb squads and fire crews remained at the scene including London Ambulance’s Hazardous Response Units.

“We have sent multiple resources to the scene including single responders in cars, ambulance crews, incident response officers and our hazardous area response team, with the first of our medics arriving in under five minutes,” the London Ambulance said.

“Our initial priority is to assess the level and nature of injuries,” it said.

Eyewitnesses outside the station reported seeing people with facial injuries. Many have been told to stay in lockdown as police evacuated the area and placed a wide zone under a police cordon.

Passenger Chris Wildish told the BBC that he saw a bucket in a supermarket bag with “low-level flames coming out of it” by the door of the rear carriage.