Detectives investigating Friday's bombing of a train in the London Underground System have arrested an 18-year-old man in connection with the investigation, London's Metropolitan Police announced Saturday.

The man was arrested by Kent Police in the port area of Dover and remains in custody. Authorities are treating the bombing as a terrorist attack.

"We have made a significant arrest in our investigation this morning," said Deputy Assistant Commissioner Neil Basu, senior national co-ordinator for Counter Terrorism Policing. "Although we are pleased with the progress made, this investigation continues and the threat level remains at critical."

Police did not identify the man.

"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," Basu said.