LONDON — Britain was hit by a 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 device exploded at 8:20 a.m. on an eastbound District Line train leaving the Parsons Green station in Southwest London.
“This was a detonation of an improvised explosive device,” Assistant Commissioner Mark Rowley of the Metropolitan Police, a top counterterrorism official, said at a news conference. He urged anyone who had seen what 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 22 people were hospitalized, several of whom had apparently been injured as panicked commuters fled. None had life-threatening injuries, and hospital officials described the victims as “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.
The National Health Service said that 19 people had been taken to three hospitals and that another three had gone on their own.
It was the fifth 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 the busiest.
Prime Minister Theresa May returned to London from her constituency in Maidenhead, west of the capital, 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 the earlier attacks, 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” had been “in the sights of Scotland Yard,” but he did not elaborate on what he meant.