An armed police operation linked to the Parsons Green Tube train explosion is taking place in the Surrey town of Sunbury-on-Thames, which is on the south-west outskirts of Greater London.

Residents of Cavendish Road are being told by police to leave their homes and retreat to a point 100m away from where the police operation is taking place.

The operation comes just hours after Metropolitan Police announced the arrest of an 18-year-old man in Dover in connection with the Parsons Green terror attack.

Sunbury-on-Thames is located approximately 15 miles south-west of central London, between the commuter towns of Kingston and Ashford, Surrey.

An improvised bomb partially exploded on a packed commuter train at 8:20am on Friday morning (15 September), leaving 29 people injured. The District Line Tube train, heading from Wimbledon to central London, had pulled into Parsons Green station moments earlier.

Surrey Police issued the following statement:

British Home Secretary Amber Rudd says it is "much too early" to tell if authorities knew of the suspect in the London subway bombing.

Rudd said Saturday it was "good fortune" that the bomb on the District Line train did not do more damage. She said it was a "serious" improvised explosive device that could have caused much more harm.

She says the arrest Saturday of an 18-year-old suspect was "very significant" and that police are making rapid progress in the investigation. Rudd said the independent Joint Terrorism Analysis Center will gauge whether to keep the country's terrorist threat level at "critical" in the coming days.