London commuter train explosion treated as 'terrorism': police

London police are treating an explosion on a rush-hour commuter train on Friday morning as a 'terror incident'.

At least 22 passengers suffered flash burns after an improvised explosive device was partially detonated on a Tube carriage at Parsons Green station in the city's south-west.

Photos taken inside the District Line train show a white plastic bucket inside a supermarket shopping bag. Flames and what appear to be wires can be seen.

The scene was evacuated and people asked to avoid the area.

The Metropolitan Police's Deputy Assistant Commissioner Neil Basu declared it a terrorist incident and a manhunt is underway to find who was responsible for placing the device, believed to be triggered by a timer.

Assistant Commissioner Mark Rowley said hundreds of police officers, working with the MI5, are investigating the incident, and patrols were being stepped up across the capital.

"We now assess that this was a detonation of an improvised explosive device," Rowley told reporters, declining to say whether the suspected bomber had been on the train.

Emergency services were called to reports of a fire at 8.20am.

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Passengers have described the chaos and confusion at the scene as people ran in terror. Eyewitnesses described a fire ball surging through the carriage.

One commuter, Richard Aylmer-Hall, said he saw several people injured, apparently trampled as they fled.

"There was a woman on the platform who said she had seen a bag, a flash and a bang, so obviously something had gone off," he said.

"Some people got pushed over and trampled on, I saw two women being treated by ambulance crews."

Fire ball

Peter, one of those injured in the incident, said his hair was singed and head burnt by a "fire ball".

"I heard a large bang on the other side of the tube train. Then this fire ball above my head has come and singed all my hair. I've got burn marks to the top of my head. Everyone just ran off the train. It was quite scary.

"There was a lot of people worse than me. I saw a gentleman in a puffer jacket and the whole back of that had been burned. He had burn marks across his facewhich were a lot worse than mine.

London Ambulance Service said a number of responders, including its hazardous area response team, were at the scene alongside the police.

St Mary's Hospital in Paddington declared a major incident to deal with the injured. None of those hospitalised are reported to have serious or life-threatening injuries.

Unhelpful

Britain's threat level remained on its second-highest rank of severe following the "cowardly attack" intended to cause significant harm, said Prime Minister Theresa May. 

She added it was not helpful for anyone to speculate on investigations after US President Donald Trump tweeted that the culprits had been "in the sights" of the police.

"Another attack in London by a loser terrorist," the tweet said. "These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!"

His comments appeared to point the finger at British police. Asked whether Trump knew something Britain did not, May said: "I never think it's helpful for anybody to speculate on what is an ongoing investigation."