London Underground blast: 18-year-old terror suspect arrested in Dover

British police hunting those behind a bomb which injured 30 people on a London train on Friday said they had arrested an 18-year-old man in a move described as significant.

"We have made a significant arrest in our investigation this morning," Neil Basu, Senior National Co-ordinator for Counter Terrorism Policing, said in a statement.

"Although we are pleased with the progress made, this investigation continues and the threat level remains at critical."

The man was arrested under the Terrorism Act in the southern port area of Dover.

Quoting security analysts SITE, The Independent reported that a "detachment" from the extremist group ISIS was responsible for the attack.

Shiraz Maher, an expert on radicalisation at Kings College London, said it was unusual for such claims to be made while the suspect was still at large.

The explosion during Friday's morning rush hour also left several passengers fleeing in panic.

Passengers suffered facial burns after the incident inside the carriage as it was sat at the overground station.

Police said a device was placed inside a bag and left by the tube carriage door before exploding towards the rear of the train, filling the carriage with smoke.

Passengers were seen "running and screaming" from the tracks.

The police were called at approximately 08:20 hrs on Friday to Parsons Green Underground Station following reports of an incident on a tube train.

The officers from the Metropolitan Police Service and the British Transport Police reached the spot along with the London Fire Brigade and the London Ambulance Service.

Hundreds of detectives and MI5 are investigating the attack. ​