LONDON — The British police said on Saturday that they had made a “significant arrest” of a suspect in the terrorist attack on a subway station in Parsons Green that injured at least 30 people and unleashed panic among fleeing passengers in London’s subway system.
The suspect, identified only as an 18-year-old man, was arrested in the port area of Dover, in the southeastern county of Kent. The police said he would be transferred to a South London police station later.
“We have made a significant arrest in our investigation this morning,” Deputy Assistant Commissioner Neil Basu, the top counterterrorism official at the Metropolitan Police in London, said in a statement.
The Islamic State claimed responsibility for the blast on Friday in a statement that said “a detachment” of its militants had carried out the attack.
Britain raised its terror threat level to “critical,” the highest level, after the attack, meaning that another assault was “expected imminently.” Mr. Basu said on Saturday that the threat level would remain that way as the investigation continued.
Ben Wallace, the security minister, told the BBC’s “Today” radio program that the homemade device used to carry out the attack had contained triacetone triperoxide, or TATP, an explosive similar to the one used in the Manchester Arena bombing in May.
The Metropolitan Police said a major hunt was still underway for any other suspects connected to the subway bombing. The police in Kent warned residents in a statement on Twitter to expect more officers and “military personnel” on the streets. Hundreds of soldiers have been deployed across strategic sites, the Metropolitan Police said.
Dover, about 75 miles from London, is one of the main ports on the English Channel for ferry services linking Britain with mainland Europe.
As part of the bombing investigation, armed officers on Saturday were searching at least one house in Sunbury-on-Thames, on the outskirts of the capital and about three miles from Heathrow Airport in London, according to The Associated Press, the BBC and other local news outlets. The authorities had put up cordons in the area and evacuated residents, the reports said.
The police confirmed that a residential address in the area had been evacuated and was being searched, but declined to say if the operation was linked to the suspect arrested.
Sunbury is 10 miles west of Parsons Green, where the explosion occurred. Residents reached by phone on Saturday said that shortly before 2 p.m., police officers arrived to seal off a residential neighborhood around Cavendish Road and Burgoyne Road.
Louise Margetts, 54, said she was returning from the supermarket to her home when she saw four police vehicles, including a police canine unit, speeding up the road.
“The officers at the back jumped out and started cordoning off the road,” she said. “They were running, and said, ‘Turn around now.’”
Mrs. Margetts, a teacher, managed to get home but the police arrived at her door soon afterward.
“They didn’t really tell us anything,” she said. “They knocked, well, hammered, on the door and said: ‘Out now. We can’t say why.’”
After the police sealed off the surrounding streets, residents were told that they had to evacuate and some were offered transport to a local rugby club, some residents said. Others were allowed to go to relatives nearby.
Barry Sutton, 53, said he saw several Surrey police vehicles on the edge of the cordon.
“The police are going house to house behind the tape, evacuating people,” he said by phone. “There was no sense of panic, but there were a lot of police; it was a proper operation.”
Sunbury is very close to London’s outermost boroughs and is served by commuter trains, but is not on the London Underground network.
Police officers have been combing the footage from CCTV on the London transportation network for clues about who had placed the bomb that exploded on Friday.
The crude explosive, wrapped in a plastic grocery bag that was concealed in a bucket, exploded at 8:20 a.m., during the morning rush, on a train at the station in West London.
The whole carriage was engulfed in flames, witnesses said, forcing dozens of passengers to trample over one another to try to exit the train. No one was killed in the attack, but at least 30 people were hospitalized for injuries including burns and fractures.
The terrorist attack on Friday was the fifth major assault in Britain in less than six months, and the first to hit mass transit since the deadly bombings of 2005 that killed 52 people. While Londoners expressed relief that no one was killed in Friday’s blast, the episode renewed fears about the threat of terror.
“This has become the new normal,” said Harry Walker, a Parsons Green resident. “We get attacked, and then we carry on, waiting in anticipation for the next one.”
“The fact that they struck Parsons Green,” he added, “which is way out from the center is them giving a very clear message: ‘We can do it anywhere at any time.’ ”
Parsons Green station reopened on Saturday, less than 24 hours after the attack, but Mr. Basu urged the public to “remain vigilant” as the authorities continued to work through the “complex investigation.”