A teenager suspected of planting the Parsons Green bomb was arrested in the departure area at the port of Dover, Scotland Yard said.
The 18-year-old was detained on suspicion of preparing a terrorist act, sparking an evacuation of the ferry terminal, Deputy Assistant Commissioner Neil Basu said.
Detectives investigating the bombing were said to be 'keeping an open mind' about whether more plotters were involved and Mr Basu said the terror threat would remain at its highest level, indicating another attack was feared.
He added: "Officers from Kent Police had to partially evacuate the port of Dover at around 11.40am this morning and this was to enable them to search the premises as a precautionary measure.
"That work is now complete and they have recovered a number of items during that search."
A separate search is under way at a house in Sunbury-on-Thames, Surrey - home to foster care couple Penelope and Ronald Jones, who received MBEs for services to children and families.
The surrounding houses were evacuated by counter-terror officers.