London (CNN) A rush-hour blast caused by an improvised explosive device on a London Underground train which injured 22 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.

"We now assess that this was a detonation of an improvised explosive device," Metropolitan Police Assistant Commissioner Mark Rowley said at a press conference Friday.

Most of the injuries appear to be the result of flash burns, he said.

Asked by a reporter if police had any information on the perpetrator, Rowley said: "It's a live investigation."

Hundreds of detectives are working on the inquiry with the support of Britain's domestic intelligence service, MI5.

Eyewitness Sylvain Pennec took this photo, he described panic as commuters escaped the carriage.

A British security source who has been briefed on the investigation told CNN that a timer was found on the device. It's clear the device was intended to cause much greater damage, the source said, but cautioned that the investigation is still in its preliminary stages.

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 to discuss the incident, Downing Street said.

Members of the emergency services work near Parsons Green underground tube station.

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.

"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 people were taken to a nearby restaurant, where some were treated by first responders.

A woman is tended by first responders near Parsons Green station.

'Explosion noise'

Another witness, software developer Sylvain Pennec, told CNN: "It was maybe 10 meters from me, the explosion. I didn't fully realize because I had my headphones on. I heard a big... 'Bam.' An explosion noise.

"When I looked I saw people starting to get scared. We were lucky because we were at the Parsons Green station and the doors were starting to open. Everyone managed to get out of the train."

Pennec said the device looked "like a big mayonnaise bucket" but he was unsure what was inside.

"I saw some people with burns, and things like that. But, from what I saw, most people were injured from the movement of the crowd," he added.

A picture taken by a member of the public outside Parsons Green Tube station shows bystanders and emergency services.

London Mayor Sadiq Khan said he was coordinating with the police and government 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."

The @metpoliceuk have confirmed that the explosion on a train at Parsons Green Station is being treated as terrorism https://t.co/4jRHedcBXt pic.twitter.com/Vbt5uCFpih — Sadiq Khan (@SadiqKhan) September 15, 2017

US President Donald Trump responded to the incident in a series of tweets.

"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!" he said.

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! — Donald J. Trump (@realDonaldTrump) September 15, 2017

He followed up with: "Loser terrorists must be dealt with in a much tougher manner. The internet is their main recruitment tool which we must cut off & use better!"

Trump then called for the implementation of his administration's contested travel ban, which bars certain people from entering the United States, saying: "The travel ban into the United States should be far larger, tougher and more specific - but stupidly, that would not be politically correct!"

Passengers evacuated

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.

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.

Members of the emergency services work outside Parsons Green Tube station on Friday.

Local businesses and residents in Parsons Green, an affluent, mainly residential area popular with young families in the borough of Hammersmith and Fulham, not far from Chelsea, offered help to those in need.

Katy Llewellyn-Jones, who lives nearby, told CNN 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.

"It's frightening to think this can happen right by your home," said Llewellyn-Jones, who had heard screams from the train.

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.

Anyone with images from the scene at #ParsonsGreen tube station is asked to upload them to https://t.co/l9dn1FQr7B pic.twitter.com/6ZzZBnlNG6 — Metropolitan Police (@metpoliceuk) September 15, 2017

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.