A 17-year-old boy has been arrested by detectives investigating last Friday's Parsons Green tube attack.
The boy was detained at an address in Thornton Heath, south London, at around 12.05am on Thursday, Scotland Yard said.
Officers have begun a search of the property where the teenager was arrested under section 41 of the Terrorism Act.
It brings the number of people being held in connection with the attack to six. They are all being held at a south London police station.
Commander Dean Haydon, head of the Met's Counter Terrorism Command, said: "This continues to be a fast-moving investigation. A significant amount of activity has taken place since the attack on Friday.
"We now have six males in custody and searches are continuing at five addresses. Detectives are carrying out extensive inquiries to determine the full facts behind the attack."
The investigation into the rush hour bombing - which injured 30 people - has seen three men aged 25, 30 and 48, detained in Newport, south Wales, a 21-year-old named Yahyah Farroukh arrested in Hounslow, west London, and an 18-year-old man detained at the Port of Dover.
A time limit on Mr Farroukh's detention is expected to expire on Thursday, while detectives have been given until Saturday to continue questioning the 18-year-old suspect.