The 18-year-old man was arrested by Kent Police in the port area of Dover this morning, Saturday, 16 September, under section 41 of the Terrorism Act, the Metropolitan Police said in a statement.
The man remains in custody at a local police station.
He will be transferred to a south London police station in due course, the police said.
Deputy Assistant Commissioner Neil Basu, Senior National Co-ordinator for Counter Terrorism Policing, said: “We have made a significant arrest in our investigation this morning. Although we are pleased with the progress made, this investigation continues and the threat level remains at critical."
“The public should remain vigilant as our staff, officers and partners continue to work through this complex investigation. We are not, at this time, changing our protective security measures and the steps taken to free up extra armed officers remain in place," Basu said.
“This arrest will lead to more activity from our officers. For strong investigative reasons we will not give any more details on the man we arrested at this stage," he said.
Detectives from the Met's Counter Terrorism Command have urged people with information about the terrorist attack to contact police.
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 on Friday.
So far, detectives have spoken to 45 witnesses and continue to receive information from the public to the confidential anti-terrorist hotline.
Earlier, terrorist group ISIS has claimed the attack through its mouthpiece Amaq.According to an official communique released by the outfit, it plans more attacks.Following the incident, threat level in the city has been raised to 'critical', London Mayor Sadiq Khan tweeted.
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