The arrest was made in the port area of Dover, where passenger ferries sail to France. The train bombing that took place at the Parsons Green underground station injured 30 people.
British police arrested an 18-year-old man in the southern port of Dover on Saturday in a "significant" development in the hunt for the people behind a London commuter train bombing that injured 30 people a day earlier.
Prime Minister Theresa May put Britain on the highest security level of "critical" late on Friday, meaning an attack may be imminent, and soldiers and armed police deployed to secure strategic sites and track down the perpetrators.
The home-made bomb shot flames through a packed commuter train during the Friday morning rush hour in west London but apparently failed to detonate fully. Daesh claimed responsibility for the attack.
"We have made a significant arrest in our investigation this morning," said 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."
The arrest was made in the port area of Dover, where passenger ferries sail to France.
The blast on the London tube train at the Parsons Green underground station was the fifth major terrorism attack in Britain this year and was claimed by Daesh.
TRT World's Christine Pirovolakis reports.
Britain deployed hundreds of soldiers at strategic sites such as nuclear power plants and ministry of defence sites on Saturday to free up armed police to help in the hunt for those behind the bombing.
The last time Britain was put on "critical" alert was after a man killed 22 people at an Ariana Grande concert in Manchester in May. Prior to that it had not been triggered since 2007.
"For this period, military personnel will replace police officers on guard duties at certain protected sites," May said in a televised statement.
"The public will see more armed police on the transport network and on our streets providing extra protection. This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses."
TRT World's Sarah Morice has more from London.