Police vehicles line the street near Parsons Green tube station in London, Britain, Sept. 15, 2017.
Police vehicles line the street near Parsons Green tube station in London, Britain, Sept. 15, 2017.

A makeshift explosive device detonated early Friday on a packed rush-hour commuter train in London, injuring at least 22 people in what police say was a terrorist attack. Emergency workers said none of the injuries were believed to be life-threatening.

Police descended on the Parsons Green underground subway station after commuters reported an explosion and a fire, causing panic and disrupting service.

The blast was the fifth major terrorist attack in Britain this year.

map of parsons green subway in london britain

British Prime Minister Theresa May called the West London attack "cowardly" and urged London residents to go about their normal routines, although she said," The threat level remains at severe."

May admonished U.S. President Donald Trump for his criticism of Britain's approach to terrorism. Trump tweeted, "Another attack in London by a loser terrorist.These are sick and demented people who were in the sights of Scotland Yard. Must be proactive!," Trump wrote without elaborating.

May responded to the tweet, telling the BBC, "I never think it's helpful for anybody to speculate on what is an ongoing investigation."

WATCH: May on device used in attack

President Donald Trump described the attack as a "very, very terrible thing" Friday and told reporters he would be soon speaking to May about the incident.

WATCH: Trump on London attack

Of the five terrorists who attacked Britain this year, at least three were known to law enforcement authorities.

London police said their investigation into Friday's attack is being supported by input from MI-5, Britain's domestic intelligence agency.

British Foreign Minister Boris Johnson appealed for calm and said it was important not to speculate.  London Mayor Sadiq Khan said the British capital "will never be intimidated or defeated by terrorism."

Chaotic scene

Shortly after the attack, witnesses described a scene of panic.

Chris Wildish, a passenger on the train, said there was a "massive flash of flames" scorching the top of the train, followed by the smell of chemicals. He said a number of schoolchildren were on the rush-hour train and they were knocked around as passengers panicked as they exited the train.  

This is an image made from video showing burning i
This is an image made from video showing burning items in underground train at the scene of an explosion in London, Sept. 15, 2017.

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

May convened an emergency meeting Friday in response to the attack.

In Photos: London subway blast

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