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British Police Make ’Significant Arrest’ Connected to London Train Blast

18-year-old man arrested Saturday in connection with terror attack on London tube.

ISIS has claimed responsibility for the attack.

An apparent bucket-bomb explosion rocked a London subway train Friday morning.

The terror attack is the sixth in Britain this year.

The blast sent 29 people to the hospital.

A manhunt is underway for perpetrators, with no arrests yet.

The threat level has been raised to "critical," meaning an attack is expected imminently.

Witnesses described a ball of flames and a loud explosion.

Police said the improvised explosive device used did not fully explode.

An 18-year-old man was arrested Saturday morning in Dover, England, in connection with Friday’s London subway terror attack, which ISIS has claimed responsibility, police announced.

The man, apprehended by Kent Police in Dover’s port area, was arrested under section 41 of the U.K.’s Terrorism Act, London Metropolitan Police said in a statement. The man remains in custody at a local police station. He will be transferred to a south London police station.

"We have made a significant arrest in our investigation this morning," Deputy Assistant Commissioner Neil Basu, Senior National Co-ordinator for Counter Terrorism Policing, said. "Although we are pleased with the progress made, this investigation continues and the threat level remains at critical."

Basu added, "“This arrest will lead to more activity from our officers. For strong investigative reasons we will not give any more details on the man we arrested at this stage.”

ISIS claimed responsibility for the attack through its propaganda news agency, Amaq.

A manhunt kicked off after an apparent bucket bomb exploded on a subway train during rush hour Friday, sending a fireball through the passenger car and sending at least 29 people to the hospital, officials said.

"This is a live investigation," Assistant Commissioner for London’s Metropolitan Police Service Mark Rowley said outside Scotland Yard, the police force’s headquarters, shortly after the explosion occurred at the Parsons Green tube station just after 8 a.m. local time.

"There are many urgent inquiries ongoing with hundreds of detectives involved, looking at CCTV, carrying out forensic work and speaking to witnesses," Rowley, the head of national counter-terrorism policing, added.

Transit officials announced early Saturday morning that the Parsons Green station had fully reopened and "and there’s good service operating on the line following yesterdays incident."

British Prime Minister Theresa May said after an emergency Cabinet meeting that the nation’s threat level has been raised to "critical." The public should remain vigilant, she said.

A British government official said the incident appears to be an isolated incident.

Eyewitnesses told ABC News that the blast happened as the train pulled into the station in the affluent, mainly residential area of West London.

"I heard a loud bang and as I looked to my right, there was a flame, a fireball came through the carriage. ... As the doors opened, people then began leaving the train straight away," said Martin Adams, a security manager for The Walt Disney Company who was riding in the subway car at the time. "I saw some flames coming from what I thought was a blue bag."

Another person who was at the Parsons Green station said she saw a number of people with what appeared to be facial burns and singed hair after the explosion.

"I saw a couple people with burns. One lady had her hair badly singed by the fire," said Sally Faulding, who witnessed the panic on the subway platform. "I also saw people injured obviously from having been stampeded on the platform because we were all running. People were falling over."

The London Ambulance Service said it transported 19 patients to area hospitals and 10 others took themselves, but none of the injuries were serious or life-threatening.

Adams said the situation was managed well by train operators and emergency crews, and that passengers remained relatively calm.

"There were no casualties at all on the train. Everybody appeared to get off," Adams told ABC News. "There was nobody laying on the platform floor. I assessed that there were no serious casualties at that time."

Police said the improvised explosive device on the train did not fully explode. No arrests have been made at this time.

The area surrounding the Parsons Green tube station was evacuated as emergency services continue to work at the scene and officers secure the remnants of the device, police said.

Apparent images of the device show wires hanging out of a white bucket.

Metropolitan Police Deputy Assistant Commissioner Neil Basu, the senior national coordinator for counter-terrorism policing, declared the event as a "terrorist incident."

London Mayor Sadiq Khan released a statement condemning those responsible for the attack.

"Our city utterly condemns the hideous individuals who attempt to use terror to harm us and destroy our way of life," Khan said. "As London has proven again and again, we will never be intimidated or defeated by terrorism."

U.S. President Donald Trump, who has clashed with Khan in the past, shared his thoughts on Twitter just before 7 a.m. ET.

Trump referred to the suspects being "in the sights of Scotland Yard," though London officials have not publicly provided any confirmation of that.

Law enforcement agencies across the U.S. said they are closely monitoring Friday’s incident in London.

New York City Police Commissioner James O’Neill said there were no known threats to the city’s subway system, but reminded passengers to remain vigilant and aware of their surroundings.

However, New York Gov. Andrew Cuomo released a statement saying he was increasing security in public transportation areas across the Empire State.

"Out of an abundance of caution, I am directing state law enforcement to increase security at vital assets across New York, including airports, bridges, tunnels and mass transit systems," Cuomo said. "The safety and security of New Yorkers is our No. 1 priority, and we remain in close contact with local and federal officials. We remain vigilant, and we stand with the people of London."