Britain has raised its terror threat level to "critical" after a bomb attack on the London subway Friday that has been claimed by Islamic State.
Prime Minister Theresa May announced Friday after the attack that the country's Joint Terrorism Analysis Center, or JTAC, has decided to raise the threat level from "severe" to "critical" — meaning that according to May, "their assessment is that a further attack may be imminent."
She said members of the military will begin aiding police, providing security at some sites not accessible to the public. May also said members of the public may see more armed police on the streets and the transport network.
Meanwhile, Islamic State has said the improvised explosive device used at Parsons Green subway station early Friday, injuring at least 29 people, was detonated by an IS-affiliated unit.
The makeshift explosive device exploded on a packed rush hour commuter train in what police call a terrorist attack. Emergency workers said none of the injuries were believed to be life-threatening. Eight of the injured have been treated and sent home.
"There was a loud shriek to my left," said Luke Walmsley, who was close to the explosion and became caught up in the panicked crowd afterward. "And I turned and there were the remnants of a flash and smoke. And then just hordes of people. The stampede had already started."
Police descended on the underground subway station at Parsons Green to investigate. A large cordon has been put up around the area and service on parts of the subway network, nicknamed "the Tube," have been suspended.
Mark Rowley, a deputy assistant commissioner of London's Metropolitan Police, told reporters the blast appeared to be deliberate. After some initial investigation, Rowley told reporters, "We now assess that this was a detonation of an improvised explosive device."
Another passenger, Chris Wildish, 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 school children on the rush hour train were knocked around by adult passengers running toward the exits.
Images posted on social media appear to show a bucket on fire that had been placed inside a plastic bag close to a rail car door.
The blast was the fifth major terrorist attack in Britain this year.
In Photos: London subway attack
May condemns attack
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."
WATCH: May on malicious intent of attacker
United Nations Secretary-General Antionio Guterres released a statement saying, "We condemn the attack and we wish a speedy recovery to those who were injured. The U.N. stands in solidarity with the people and government of the United Kingdom."
U.S. President Donald Trump called Prime Minister May on Friday to convey his sympathies, the office of the White House press secretary said in a statement.
The statement said President Trump "pledged to continue close collaboration with the United Kingdom to stop attacks worldwide targeting innocent civilians and to combat extremism."
WATCH: Trump on London attack
Earlier, the British prime minister admonished Trump for his initial reaction to the attack. Trump had 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!"
May responded to the tweet, telling the BBC, "I never think it's helpful for anybody to speculate on what is an ongoing investigation."
London police said their investigation into Friday's attack is being supported by 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."
May convened an emergency meeting Friday in response to the attack.