An 18-year-old man has been arrested on suspicion of terror offences in connection with Friday’s attack on a London Tube.
The man was detained in the port area of Dover on Saturday by Kent Police and is being held at a local station.
Metropolitan Police Deputy Assistant Commissioner Neil Basu said the arrest was “significant”, but the terror threat level remains at “critical”.
Thirty people were injured after the explosion on a train at Parsons Green.
Most people were treated for minor injuries and have been released, the London Ambulance Service said, but three people remain in hospital.
Home Secretary Amber Rudd will be chairing a meeting of the government’s Cobra emergency committee later.
Mr Basu said the public should remain vigilant, as the force was not changing its “protective security measures” and extra armed officers were still being deployed.
He 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.”
The man is due to be moved from Kent to a south London police station later.
The Islamic State group has said it was behind the bomb, which detonated at 08:20 BST on Friday.
It is understood the device had a timer, but the BBC’s security correspondent Frank Gardner said the bomb appeared not to have gone off properly.
Had it worked as intended, it would have killed everyone around it and maimed everyone in the train carriage for life, he said.
The Met’s Assistant Commissioner Mark Rowley said it was “very routine” for the Islamic State group to claim the attack, whether in contact with those involved or not.