LONDON — A rush hour blast caused by an improvised explosive device on a London Underground train which injured 18 people is being treated as terrorism by London’s Metropolitan Police.

None of the injured are thought to be in a serious or life-threatening condition, according to the London Ambulance Service.

The blast occurred at about 8:20 a.m. local time (3:20 a.m. ET) Friday on a busy commuter line into central London.

Deputy Assistant Commissioner Neil Basu, the senior national coordinator for counterterrorism policing, has declared it a terrorist incident, the Met Police said in a statement.

Downing Street tweeted a message from Prime Minister Theresa May, saying: “My thoughts are with those injured at Parsons Green and emergency services who are responding bravely to this terrorist incident.”

May will chair an emergency COBRA meeting on Friday after the London underground terror incident, Downing Street said.

Parsons Green station is closed, a large security cordon has been put in place and large numbers of emergency responders, including armed police officers, are at the scene.

London Ambulance Service said a number of responders, including its hazardous area response team, were present.

“We can confirm we have taken 18 patients to hospital following the incident at Parsons Green underground station,” Natasha Wills, the ambulance service’s assistant director of operations, said.

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“Suddenly, there was this boom,” eyewitness Gustavo Vieira told CNN. “Everyone shouting and screaming….We were just leaving the carriage [when the explosion happened]… Just heard and everyone starting running….And I didn’t look back.”

He said everyone was taken to a nearby restaurant, where some were treated by first responders.

‘Flames all around’

Software developer Sylvain Pennec told the UK’s Press Association news agency he heard a “boom and when I looked there were flames all around.”

“People started to run but we were lucky to be stopping at Parsons Green as the door started to open.”

Richard Aylmer-Hall, 52, a media technology consultant, described “panic” on the train in an interview with PA.

“There was panic, lots of people shouting, screaming, lots of screaming,” he said.

“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. “I saw two women being treated by ambulance crews.”

St Mary’s Hospital, which is near Parsons Green station, has declared a “major incident,” according to its website.

London Mayor Sadiq Khan said he was coordinating with the police and authorities and urged Londoners to “remain calm and vigilant.”

“Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life,” he said. “As London has proven again and again, we will never be intimidated or defeated by terrorism.”

Niyi Shokunbi, a 24-year-old fitness instructor who was on the train, told CNN there had been scenes of panic as people scrambled to get off the packed train on to a small station platform. There were children and elderly people among them, he said.

Video clips filmed by Shokunbi showed one person lying on the ground being helped by an emergency responder and another lying on a stretcher.

Passengers evacuated

James Macnaughton, who was stuck on a train behind the one on which the incident occurred, told CNN that he and other passengers had been evacuated in small groups by walking down the train line.

“There is a really strong police presence, helicopters overhead, lots of dogs,” he said.

The security cordon around the station, which is above ground, has been extended and emergency vehicles could be seen continuing to arrive well over an hour after the incident. Parsons Green is an affluent, mainly residential area popular with young families in the borough of Hammersmith and Fulham, not far from Chelsea.

“I was looking up at the train and I heard screams,” Katy Llewellyn-Jones, who lives nearby, told CNN.

Llewellyn-Jones said she had given shelter in her apartment to a woman who had lost her shoes in the confusion and had asked for somewhere safe to go.

“She said as she was running off, people didn’t realize [what had happened] and people were still trying to get on the Tube,” Llewellyn-Jones said.

“It’s frightening to think this can happen right by your home.”

Train services between Earl’s Court and Wimbledon on the District Line have been suspended, Transport for London said. Customers are advised to take alternative routes.

The Met Police have appealed for any witnesses to send in images and video from the scene.

The UK terror threat level is currently “severe,” the second highest level, meaning an attack is highly likely.

There have already been four confirmed terror attacks in England this year, three of them in London and one in Manchester.

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