A second man has been arrested by Scotland Yard's Counter-Terrorism Command in connection with the bombing at a London Underground train, police said on Sunday.
Officers arrested a 21-year-old just before midnight in Hounslow, West London.
This is the second arrest after an 18-year-old man was arrested by Kent police in the port area of Dover on Saturday morning.
Both suspects are being held under Section 41 of the UK's Terrorism Act and are being questioned at a south London police station.
"The Metropolitan Police and its partners across the Counter-Terrorism Policing Network have been working around the clock and through the night to identify, locate and arrest those responsible for this cowardly crime," Neil Basu, the Met Police's Senior National Co-ordinator for Counter Terrorism Policing, said in a statement.
"At this stage we are keeping an open mind around whether more than one person is responsible for the attack and we are still pursuing numerous lines of enquiry and at a great pace," he said.
Thirty people are known to have been injured during the attack in which an improvised explosive device was detonated on a Tube train at Parsons Green underground station at around 08:20hrs local time on Friday.
Following the first arrest yesterday, Met Police counter-terrorism specialist firearms officers had evacuated buildings as they began searches at a residential address in Sunbury, Surrey, south-east England.
The search remains ongoing as it emerged that the home belongs to an elderly British couple honoured with an MBE by Queen Elizabeth II in 2010 for their efforts at fostering hundreds of refugee children.
The 18-year-old arrested is believed to have been one of the children fostered by 88-year-old Ronald Jones, 88, and 71 -year-old Penelope Jones.
Meanwhile, the UK terror threat level remains 'critical', meaning an attack is expected 'imminently'.
The Islamic State (ISIS) group has said it was behind the bomb but Met Police's Assistant Commissioner Mark Rowley said it was "very routine" for ISIS to claim the attack, whether in contact with those involved or not.
The blast, which is being described as a "bucket bomb" sent a "fireball" through the Tube causing burn injuries to many commuters.
The main device, which had been fitted with a crude timer using shop-bought fairy lights, failed to detonate, meaning hundreds of people were spared death and serious injury.