Police have arrested an 18-year-old man on suspicion of a terror offence in connection with Friday’s attack on a London Tube train.
Armed police are continuing to search a house in Sunbury-on-Thames, Surrey.
The arrest is “very significant” and the terror threat level remains “critical”, Metropolitan Police Deputy Assistant Commissioner Neil Basu said.
The man was arrested in the departure area of Dover port on Saturday and has been moved to a London police station.
Thirty people were injured after the explosion on a train at Parsons Green.
The arrest sparked an evacuation of Dover port and a number of items were recovered in a police search, Mr Basu said, adding that detectives were “keeping an open mind” as to whether more people were involved in the Tube attack.
Residents of houses surrounding the Sunbury-on-Thames property have been allowed to return home after being evacuated by officers earlier on Saturday.
Locals said the property being searched by police is occupied by married couple Ronald Jones, 88, and Penelope Jones, 71.
The couple are known for fostering many children and young people over several decades, and in 2010 were rewarded by the Queen by being made MBEs.
One neighbour, Anna Wilkins, told BBC News she was “shaking like jelly” after being told by armed police to leave the home she shares with her partner and two young children.
“As I walked out of my house I saw three armed officers,” she said. “One had a balaclava on, all I could see were his eyes, I was really frightened.”
Many residents had been evacuated to Staines Rugby Club where one local, Dan Norcott, said the council had begun making preliminary arrangements for hotel accommodation.
After a meeting of the government’s Cobra emergency committee earlier, Home Secretary Amber Rudd said it was “much too early” to say whether those behind the attack were known to authorities, adding: “The operation is ongoing”.
The terror threat level was raised to critical, meaning an attack may be imminent, on Friday evening.
Met Police Commissioner Cressida Dick described the attack as “cowardly and indiscriminate”.
Ms Dick, who travelled by Tube 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.”
Operation Temperer, which involves the use of the military was put in place after the threat level was raised and is being stepped up gradually.
Home affairs correspondent Danny Shaw said it is mainly focused on London and the south-east of England and is 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 this weekend.
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.