An 18-year-old man has been arrested on suspicion of a terror offence in connection with Friday’s attack on a London Tube train.
The man was held in Dover on Saturday and taken to a Kent police station – he will be moved to south London later.
The arrest is “significant” but the terror threat level remains at “critical”, Metropolitan Police Deputy Assistant Commissioner Neil Basu said.
Armed police are now at an address in Sunbury, Surrey, the BBC understands.
Thirty people were injured after the explosion on a train at Parsons Green.
Met Police Commissioner Cressida Dick described the attack as “cowardly and indiscriminate”.
Ms Dick, who travelled by Tube to Waterloo station before joining Met officers patrolling the South Bank, said: “London has not stopped after other terrible attacks and it will not stop after this one.”
Home Secretary Amber Rudd chaired a meeting of the government’s Cobra emergency committee and it concluded the UK terrorist threat level should remain at “critical”.
It had been raised to that level – which is the highest and means an attack may be imminent – on Friday evening.
The public should remain vigilant, Mr Basu said, as the force was not changing its “protective security measures” and extra armed officers were still being deployed.
Operation Temperer, which involves the use of the military and was put in place after the threat level was raised, is being stepped up gradually.
Home affairs correspondent Danny Shaw said it is focused on London and the south-east of England and is being described as “light-touch”, and not on the same scale as what was seen after the Manchester Arena attack.
Mayor of London Sadiq Khan said “significant” police activity would continue over the weekend and thanked police, adding: “They are there to keep us safe”.
More armed police officers will be present at London Underground stations, as well as at stations across England, Scotland and Wales, British Transport Police Assistant Chief Constable Robin Smith told the BBC.
Most people caught in the blast were treated for minor injuries and have been released, NHS England said, but three people remain in Chelsea and Westminster hospital in central London.