The explosion occurred at 8:20 a.m. on a District Line train as it left the Parsons Green station in Southwest London. At least 18 people were taken to hospitals. Several of them were apparently injured as panicked commuters fled. None had life-threatening injuries.
“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.
The authorities were cautious and circumspect for about two hours after the explosion, saying only that an “incident” occurred.
But then the Metropolitan Police announced that Deputy Assistant Commissioner Neil Basu, the senior national coordinator for counterterrorism policing, “has declared it a terrorist incident.”
The police added: “Too early to confirm cause of fire — now subject to the investigation that is underway. Station remains cordoned off.”
Britain has been hit by several deadly terrorist attacks this year: 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, has been a face of resolve following deadly attacks in the capital this year.
“Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life,” Mr. Khan wrote in a statement on Facebook. “As London has proven again and again, we will never be intimidated or defeated by terrorism.”
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 burnt 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.”
A reporter for the commuter daily Metro who was at the scene reported that several people had been badly burned. There were numerous reports of people with cuts or bruises as people raced to leave the train. One woman was taken away on a stretcher, with burns on her face, according to the BBC.