Twenty-nine people were injured by an improvised explosive device which detonated on a tube train on Friday morning. British authorities have classed the incident as a terrorist attack.
READ MORE: UK terrorism threat level raised from ‘severe’ to ‘critical’ – PM May
“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,” 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.”
“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,” Deputy Assistant Commissioner Neil Basu, Senior National Coordinator for Counter Terrorism Policing, said.
“This arrest will lead to more activity from our officers,” he continued. “For strong investigative reasons we will not give any more details on the man we arrested at this stage.”
READ MORE: First moments after Parsons Green explosion caught on camera (VIDEOS, PHOTOS)
Basu advised the public to “remain vigilant” and said their “complex investigation” into the attack is ongoing.
“We are not, at this time, changing our protective security measures and the steps taken to free up extra armed officers remain in place,” he added.
Also in connection to the investigation, armed police are carrying out a search-and-evacuation operation on Cavendish Road in the town of Sunbury-on-Thames in Surrey, ordering local residents to move behind a 100-metre cordon while they search a residential property.
"Cordons are being put in place at a 100 metre radius to facilitate the Metropolitan Police Service’s operation, which is being supported by colleagues from Surrey Police," they said.
Following the attack, Prime Minister Theresa May announced the UK terrorism threat level was raised from ‘severe’ to ‘critical.’ ‘Critical’ is the highest threat level.
“Military personnel will replace police officers on guard duties at certain protected sites that are not accessible to the public,” May told media.
ISIS claimed responsibility for Friday’s explosion on a District Line train at Parsons Green station in London.
An improvised explosive device was left on the train in a Lidl bag. It partially detonated during rush hour.
Police identified the attacker using CCTV footage and a manhunt was underway Friday. In their statement on the arrest, the Met said that investigators have spoken to 45 witnesses and received over 70 images and videos from the public.
The attack is the fifth to take place in Britain in the last six months.