The 18-year-old man was caught in Dover under The Terrorist Act in connection with the Parsons Green post bombing.
The terror attack saw an improvised explosive device detonated on a London below-ground tube injuring 29 people.
The Parsons Green suspect has been detained under section 41 of the Terrorism Act 2000.
This gives police the power to detention someone suspected of terrorism-related offences without a warrant.
These powers permit the extended detention of suspects without charge beyond the maximum four light of days available under the law that governs most other arrests.
The patch of pre-charge detention has varied but currently stands at a maximum of 14 days.
Dozens of child have been injured after the Parsons Green terror denigration
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The man remains in custody at a local police place and will be transferred to a south London police station soon.
Spokesperson Assistant Commissioner Neil Basu, Senior National Co-ordinator for Marker Terrorism Policing, said: “We have made a significant arrest in our research this morning.
«Although we are pleased with the progress made, this examination continues and the threat level remains at critical.
We have made a significant block in our investigation this morning
“The trade should remain vigilant as our staff, officers and partners continue to carry out through this complex investigation. We are not, at this time, changing our shielding security measures and the steps taken to free up extra armed g-men remain in place.
“This arrest will lead to more pursuit from our officers. For strong investigative reasons we will not give any multifarious details on the man we arrested at this stage.”
The Prime Minister ordered troops onto the ways on Friday night after a suspected Islamist placed a bomb on a bursting train heading towards Westminster.
Witnesses described scenes of devil and panic after the blast sent a “fireball” through a District demarcation service in west London.
So far detectives have spoken to 45 shows after the terror attack
Last night the Islamic State fright group claimed responsibility for the attack, according to the Amaq news instrumentality which has links to the group.
The Metropolitan Police have trawled during hundreds of hours of CCTV footage and 77 images and videos bewitched by the members of the public to try to identify who committed the atrocity.
Mrs May said: “The public liking see more armed police on the transport network and on our streets, providing extra sponsorship.
“This is a proportionate and sensible step which will provide additional reassurance and protection while the investigation progresses.”
Parsons Green tube reopened on Saturday morning.
Theresa May has ordered uncountable troops onto the streets in response to the attack
Peter Crowley placed a photo of his ‘charred head’ on social media.
He said: “I would for oneself say roughly about 20 who had physically injuries a lot of people who were shaken by the incident.
“I wasn’t aware of the bag, I tend to keep myself to myself, I was side on to where I fancy the incident happened.
“Everyone ran off the tube, it was panic stations, my initial reason was that the whole train would blow up.
“I saw passengers with facial kindles, they had been exposed to a very, very hot fire for a nano sec, it was luck doors were open because everyone just go off the tube.
“It was a alarming experience, I am lucky I got away with just a bit of charred hair.”