Two men have been arrested in relation to the terror attack at Parsons Green Tube station.

A 21-year-old man was arrested in Hounslow, at about 11.50pm on Friday, September 15, and an 18-year-old man was arrested in Dover at about 7.50am on Saturday, September 16.

During the investigation, police have evacuated the Port of Dover and a house in Sunbury, Surrey, to carry out searches, and found “a number of items” in Dover.

Police said these searches have given them a better understanding of how the bomb was made.

The Joint Terrorism Analysis Centre at MI5 has reduced the threat level from critical to severe, which means an attack is highly likely.

Assistant commissioner Mark Rowley, national lead for counter terrorism policing, said: “Members of the public will be wondering what that means in terms of their travel into work [on Monday]. For practical and precautionary reasons we made the decision that the increased resources will continue for the beginning of this week. So the public will still see that high level of policing presence – some armed, some unarmed.

“The military support we have had in place under Operation Temperer will start to phase out as we move through the coming week. There are detailed and well-rehearsed plans in place to ensure a smooth transition from Operation Temperer, and military personnel will remain at readiness to deploy in support of the police should future security situations require.

“I want to stress however, that specialist teams, well-rehearsed at dealing with major incidents, continue to provide a strong visible presence throughout the capital and across the UK.”

Thirty people were injured in the attack, which happened at about 8.20am on Friday, September 15, on a District Line train.

A make-shift bomb exploded when the train was at the station.

Police have spoken to more than 100 witnesses so far, and have received photographs and video footage from the public.

Anyone with information is urged to call the Anti-Terrorist Hotline on 0800 789 321, visit gov.uk/act, or, in an emergency, always call 999.