LONDON — Britain was hit by another terrorist attack on Friday morning, when a crude device exploded on a crowded London Underground train, injuring commuters, sowing panic, disrupting service and drawing a heavy response from armed police officers and emergency workers.
The explosion occurred at 8:20 a.m. on a District Line train as it left the Parsons Green station in Southwest London.
“This was a detonation of an improvised explosive device,” Assistant Commissioner Mark Rowley, a top counterterrorism official, said at a news conference. He urged anyone who saw what had happened, or had taken photos or videos of the bombing, to come forward.
The authorities immediately beefed up security around the transit system, as hundreds of police officers and detectives combed the scene for clues.
At least 18 people were taken to hospitals, several of them apparently injured as panicked commuters fled. None had life-threatening injuries, and hospital officials described the victims as the “walking wounded.”
“The train was packed, and I was down the other side of the carriage standing up, looking at my phone and then I heard a big boom and felt this heat on my face,” said Natalie Belford, 42, a hairdresser and beautician who was on the train. “I ran for my life, but there was no way out. The doors were full of people and the carriage was too packed to move down.”
A photo widely circulated on social media showed a white bucket inside a bag, with wires and flames coming out of it.
Passengers described seeing a wall of fire. One woman with burns was taken away on a stretcher, and several others were cut or bruised as panicked commuters fled the train and the elevated station.
At Chelsea and Westminster Hospital, a boy in school uniform was wheeled through the foyer red burn marks on his forehead by a man and by woman. She confirmed that the boy was hurt in the attack, but declined to say more, saying she felt traumatized. The boy appeared alert.
A hospital spokeswoman, Ayshea Richards, said that a “small number” of injured people had been admitted to the hospital.
“The majority were what we would call ‘walking wounded,’” she said. “We are still in a ‘major incident’ mode.”
For two hours after the explosion, the authorities were cautious and circumspect, saying only that an “incident” occurred.
It was the fifth major terrorist attack in Britain this year, following
a vehicular and knife attack near Parliament in March, a suicide bombing at a rock concert in Manchester in May, and a van and knife attack around London Bridge and a van attack outside a London mosque, both in June.
Taken together, the terrorist violence has been the deadliest on British soil since July 7, 2005, when suicide bombers set off explosions on three subway cars and a double-decker bus in London, killing 52 people and injuring scores of others.
The new attack immediately revived concerns that militants might be targeting the Underground, commonly known as the Tube — the world’s oldest subway system and one of its busiest.
Prime Minister Theresa May was monitoring the situation from 10 Downing Street and summoned a meeting of the government’s emergency committee, known as Cobra, for the afternoon. Boris Johnson, the foreign secretary, appealed for calm.
The city’s mayor, Sadiq Khan, a face of resolve following deadly attacks in the capital this year, issued a defiant statement on Facebook.
“Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life,” Mr. Khan wrote. “As London has proven again and again, we will never be intimidated or defeated by terrorism.”
From the United States, President Trump weighed in on Twitter, saying the bombing was the act of a “loser terrorist.” He said that “sick and demented people” were “in the sights of Scotland Yard,” but he did not elaborate on what he meant.
Ms. Belford, the hairdresser, said she was knocked over twice, and showed a reporter her ripped tights and bloodied knee.
“I knew it was a bomb when I saw people with charred hair and burned faces,” Ms. Belford said. “This has got to be terrorism — a bag full of explosive materials don’t just appear on a train by accident.”
Adam Davis, a 23-year-old student, said he was in the train car where the explosion occurred.
“I had my headphones on, then I felt a kind of vibration, followed by a wave of heat, and I looked down and the whole carriage was in flames,” he said. “I just got up and ran, but the carriage door was jammed with people. Everyone was screaming and trying to get out, people had blood on them everyone was pushing. It was like a stampede.”
He added: “I didn’t see any suspicious-looking people, or the bucket that’s on the media. I just saw flames and you think the worst. You think bomb. Terrorism.”
Service on the District Line between Edgware Road and Wimbledon was suspended.
The Parsons Green station, which opened in 1880, is in Hammersmith and Fulham, an inner London borough that is home to several corporate headquarters and soccer clubs. Residents in the neighborhood described scenes of panic.
“I didn’t hear anything but I saw a big crowd of people running and screaming outside my window,” said one woman, Nita Alvi, who lives near the station. “I went to try and help, but everyone was running so fast. Some of them were holding their heads, some were limping. One older woman fell to the floor.”
She added: “My first thought was that it was a stabbing but people were shouting that it was a bomb.”
Roy Ramm, a former commander of specialist operations at Scotland Yard, said that police would be undertaking a comprehensive forensic examination of the train and the device and combing through evidence to determine quickly what happened, and to try to determine if there was an ongoing threat.
He said that the London transport system was monitored by thousands of CCTV cameras and that police would be carefully examining footage to try and find clues as to when the bag was left on the train and by whom. “The police will also ask witnesses the age-old question: Did anyone see what happened?” he said. “They will be investigating what the detonation mechanism was and going through CCTV footage to see who might be behind it.”
Mr. Ramm said the police would also be looking for any communications of a bomb threat before the attack, not just in the last 24 hours, but in recent weeks, and waiting to see if anyone claimed responsibility. As part of the investigation, he said, police would be working with security services to see if any terrorist cells or police targets under surveillance had been observed in Parsons Green or the surrounding area.
Mr. Ramm noted that Parsons Green, a largely residential area, had little symbolic resonance in London, making it an odd place for a terrorist to plant a device, and raising questions as to whether whoever planted it intended for it to go off where it did.
“Parsons Green is not emblematic or symbolic, and I think that will be a puzzlement for investigating officers, who will ask: Was it intended to be detonated or did it go off there by accident?” he said. “If you look at a list of target areas in London, Parsons Green would not be in the top 100.”