UPDATE: A second man has been arrested by detectives investigating the terrorist attack at Parsons Green station on Friday.
The 21-year-old was arrested in Hounslow at 11.50pm on Saturday, the Met's Counter Terrorism Command said.
He was taken to a south London police station and remains in custody.
On Saturday an 18-year-old man was arrested in connection with the bombing on suspicion of a terror offence.
Kent Police detained the man in the port area of Dover.
Britain is on the highest terror alert with the military set to bolster security as police hunt the Parsons Green bomb "suspects".
Prime Minister Theresa May has raised the threat level to critical, meaning another attack is expected imminently.
On request from the police, and with authorisation from the Defence Secretary, the first phase of Operation Temperer is being partially enacted.
A manhunt is underway after the improvised explosive device (IED) sent a fireball through a packed London Underground train carriage in south west London, during the Friday morning rush-hour.
The Prime Minister said:
"Military personnel will replace police officers on guard duties at certain protected sites which are not accessible to the public".
She said: "The public will see more armed police on the transport network and on our streets, providing extra protection.
"This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses."
An MOD spokesperson said: "At the request of the police, we are moving quickly to deploy up to 1,000 personnel from across the three services to provide security at sites across the UK.
"This will free up extra armed police to patrol the streets and protect the public."
Assistant Commissioner Mark Rowley suggested there may have been more than one person involved stating that police were "chasing down suspects".
In a statement outside Scotland Yard, the UK's most senior anti-terror police officer said:
"Somebody has planted an improvised explosive device on the Tube - we have to be open-minded at this stage about him and about potential associates."
The Islamic State (IS) terror group has claimed responsibility for the attack through its Amaq news agency, according to the US-based Site Intelligence.