By Laura Smith-Spark, Erin McLaughlin and Nina dos Santos, CNN
(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) Fridayon 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.
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.
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.
Parsons Green station is closed, a wide security cordon has been put in place and large numbers of emergency responders, including armed police officers, are at the scene.
The number of people hospitalized after the explosion is now 22, the National Health Service said in a statement.
"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.
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.
London Mayor Sadiq Khan told CNN there would be "an enhanced police presence" across London's public transport system and urged people to be "vigilant and alert" -- but not alarmed.
"What we've seen in the last period across Europe is attempts by evil and cowardly individuals to kill, to injure and to disrupt our way of life and what we're not going to do is allow them to change how we live our lives, to be pluralistic, to be respectful," he said.
"We will do all we can to keep our city, to keep Londoners, to keep visitors, to keep businesses safe."
"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 tweeted.
A police spokesperson told CNN that Trump's comment is "pure speculation given we don't know who is involved. Any speculation is unhelpful."
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.
Local businesses and residents in Parsons Green, an affluent, mainly residential area popular with families in the borough of Hammersmith and Fulham, not far from Chelsea, offered help to those in need.
Katy Llewellyn-Jones, who lives nearby and heard screams from the train, told CNN she had given shelter 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.
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.