Police have arrested a second man in connection with the Parsons Green tube attack - as detectives continue to search a foster home.
The arrest, late on Saturday, followed one earlier that day when officers detained an 18-year-old man at Dover.
On Sunday, Scotland Yard said that a 21-year-old man had been arrested in Hounslow, south-west London, shortly before midnight.
The Metropolitan Police he was detained under section 41 of the Terrorism Act.
Police said they are searching a residential address in Stanwell, Surrey, in conjunction with the second arrest.
It comes as officers continue their raid on a respected foster couple's property in Surrey, during which a whole road was evacuated.
Counter-terror chiefs said they were "keeping an open mind" as to the number of those involved in the plot.
The UK's terror threat remains at critical - its highest level - in the wake of the Friday morning rush-hour attack.
Thirty people were injured when improvised explosive device caused a fireball on a District Line carriage.
Following a flurry of police activity on Saturday, the number of arrests now stands at two.
Witnesses described seeing the 18-year-old detained by investigators in the departure area of Dover ferry port, which was temporarily evacuated.
The second arrest came as police continued to search a residential address in Sunbury-on-Thames.
Residents on Cavendish Road were forced to evacuate as police raided the address of Penelope and Ronald Jones - a respected foster couple.
The pair were awarded MBEs in 2009 for services to children and families, and are said to have fostered up to 300 children over 40 years.
The 71 and 88-year-old were described as "beautiful people" by neighbours.
One person labelled them great pillars of the community", adding: "They do a job that not many people do."
Police described the initial arrest as "significant", but insisted its investigation was ongoing.
With the terror threat remaining at its highest level, up to 5,000 British troops are able to be deployed to help free up police.
Deputy Assistant Commissioner Neil Basu previously said police were "keeping an open mind" about the perpetrators.
He added: "If there are other people responsible it's our job to find them and that is part of the reason that we are remaining at critical threat."
Of the 30 people injured in the attack, all but one have been discharged from hospital.
Just one person is being treated at Chelsea and Westminster Hospital, which has a specialist burns unit.
- Anyone with information should call the Anti-Terrorist Hotline on 0800 789 321