An 18-year-old man has been arrested in connection with Friday's terror attack on a London Tube train.
The man was detained in the port area of Dover on Saturday morning by Kent Police.
He was arrested under section 41 of the Terrorism Act and is being held at a local police station.
Metropolitan Police Deputy Assistant Commissioner Neil Basu said the arrest was "significant", but the terror threat level remains at "critical".
ISIS has claimed responsibility for the terror attack on a crowded London subway train Friday morning through its propaganda news agency, Amaq.
A manhunt is underway in London 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.