POLICE have arrested a 21-year-old man on the outskirts of London, hours after an 18-year-old was caught at Dover.
A 21-year-old man was arrested in Hounslow, in outer west London, at 11.50pm on Saturday night local time, the Met said.
The suspect was taken to a south London police station for questioning and remains in custody, police revealed on Sunday.
The latest arrest came several hours after the police arrested the 18-year-old suspect and raided the elderly couples’ home where he is believed to have stayed.
The Met’s deputy assistant commissioner, Neil Basu, earlier said investigators were “keeping an open mind” about how many people were behind the terror plot.
The 18-year-old — the youngest to be arrested over a terror attack in the UK — is reported to be a refugee fostered by an elderly couple who were honoured by the Queen.
He is being quizzed by cops after he was seized in Dover, The Sun reports, over Friday’s London Tube attack.
The arrest is “very significant” and the terror threat level remains “critical”, Metropolitan Police Deputy Assistant Commissioner Neil Basu said.
Police have confirmed the teenager they arrested in Dover is who they suspect of planting the bomb. But they have not ruled out other possible accomplices and have kept the highest terror alert in place.
Deputy Assistant Commissioner Neil Basu said a number of items were seized in a search of the premises where he was arrested.
Another house raided in Cavendish Road, Sunbury-on-Thames in connection with the 18 year old’s arrest is believed to belong to Penelope and Ronald Jones, who have cared for 268 foster children including refugees.
The elderly couple from Surrey have been honoured by the Queen for taking in so many children in need.
Neighbours told The Sun that the couple, who have six children of their own, had started taking foster children again to help refugees from Syria and Iraq.
Alison Griffiths, a Surrey county councillor who knows Mr and Mrs Jones, said they had two young people staying with them — an 18-year-old and a 22-year-old, according to The Sun.
She described Mr and Mrs Jones as “great pillars of the community”, adding: “They do a job that not many people do.”
Ms Griffiths, 42, said the couple may not be able to return home for days as police continue to search their house.
Mr Jones, 88, and Mrs Jones, 71, have reportedly helped Surrey County Council with its Syrian vulnerable persons relocation scheme.
One neighbour told The Sun the pair currently fostered two children and one of them had been arrested two weeks ago.
“All I know is that they have two boys at the moment, both are foreign,” Serena Barber, who lives behind the property, told The Sun.
“I know about two weeks ago he was arrested by police at Parsons Green, for what I don’t know and returned back to Penny and Ron.
“After that Penny said she was going to have to stop caring for him, she couldn’t handle him.”
Neighbours who were evacuated before the raid in Surrey have spoken of their shock at being rushed out by police.
Cavendish Road resident Alan Clarke said armed police had given him a matter of minutes to evacuate.
“I feel very shocked. It does make me feel unsafe to be honest,” he said.
“I was asleep on the sofa when I heard a bang on the window and this armed police man all balaclavered up said ‘please can you evacuate’.”
More than 6 hours later, the homes remain vacant, with residents unsure when they will be permitted back inside.
Some families were permitted to return to their houses briefly, under police supervision, to retrieve pets who had been left on their own in the rush of the evacuation.
Police thanked the families of the small suburb for their patience while they carried out the “complex investigation” and said they hoped residents would be let back into their houses by the end of the night (Saturday night).
She told the Press Association that police gave her “one minute” to get her children and leave her home.
“I was in my house with my children and there was a knock at the door from the police,” she said.
“They told me to leave. They said ‘you have one minute to get out of the house and get away’,
“I just got out, I got my three children and we left the house and the street. We didn’t know what was going on”.
Earlier, armed counter-terrorism police raided a home in Sunbury, Surrey, as part of the investigation into the London bombing.
About 80 officers were involved in the mass operation at about 1:40pm local time (10:40pm AEST) and a large number of houses surrounding the area have been evacuated.
“Police officers have evacuated and are searching a residential address in Sunbury, Surrey,” the Metropolitan Police said in a statement.
“Residents in the buildings immediately surrounding the address are also being evacuated as a precautionary measure.”
Cordons have been put in place in a 100 metre radius around the property.
It comes as police also evacuated an underground train station just one stop away from Parsons Green station overnight.
Transport for London confirmed that Fulham Broadway station has been closed while “we respond to a security alert”.
Photographs have been posted to social media showing crowds of people being ushered by police outside the tube station, about 3.30pm on Saturday (12:30am Sunday AEST).
About an hour later police confirmed the security alert at Fulham Broadway has been deemed “not suspicious”.
Authorities are expecting to reopen the Vue cinema and Fulham Broadway station shortly.
It is not yet clear what the nature of the alert was and why the station and shopping centre was evacuated.
London’s terror threat level was escalated to “critical” after a homemade bomb exploded on the train at Parsons Green, leaving 29 people injured on Friday morning.
Reports suggest police believe the 18-year-old man they earlier arrested at Dover is the person who planted the bomb on the train.
Home Secretary Amber Rudd hosted another emergency Cobra committee to discuss the developments.
However, the terror alert remained at the highest level of “critical” after the meeting, meaning another attack is still expected.
Ms Rudd said the police operations were ongoing after the “significant” arrest.
“This is a very significant arrest, the police have made very good progress, but the operation is ongoing and we will have to wait to hear from them about any more information,” she said.
London hospitals have updated the number of people injured in the terror attack to 30. All but three of those people have been released from hospital and none are believed to have life-threatening injuries.