Updated: 15.15
British police investigating a bombing on the London Underground are carrying out an armed operation in Surrey.
The explosion on a train at Parsons Green injured 30 people.
Officers are understood to be at a residential property in Cavendish Road in Sunbury-on-Thames.
The street has been evacuated as a precautionary measure, according to police.
Police have set up cordon at a 100 metre radius to help the operation.
No arrests have been made at the scene.
The UK Home Secretary Amber Rudd described the earlier arrest of an 18-year-old man in connection with Friday morning's attack as "very significant", but added "the operation is ongoing".
The earlier arrest was made by Kent Police in the port area of Dover on Saturday morning under the UK's Terrorist Act.
Police say the man remains in custody at a local police station.
He is to be transferred to a London police station in due course.
Deputy Assistant Commissioner Neil Basu said said: "We have made a significant arrest in our investigation this morning.
"Although we are pleased with the progress made, this investigation continues and the threat level remains at critical.
"The public should remain vigilant as our staff, officers and partners continue to work through this complex investigation.
"We are not, at this time, changing our protective security measures and the steps taken to free up extra armed officers remain in place."
Detectives say they have so far spoken to 45 witnesses and continue to receive information from the public.
They have also been sent 77 images and videos relating to the incident following an appeal.
In this aerial image made from video, emergency workers help people to disembark a train near the Parsons Green Underground station after an explosion | Image: AP/Press Association Images
The British Prime Minister Theresa May announced the UK's terror threat level had been raised to its highest level of critical, which means an attack is expected imminently.
"The public will see more armed police on the transport network and on our streets, providing extra protection," Mrs May said.
"This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses."
She said British military personnel will also replace officers "on guard duties at certain protected sites which are not accessible to the public".
Investigators are also examining the bomb, which is understood to have had a timer.
The Mayor of London, Sadiq Khan, said: "The national threat level has now been raised to critical after the evil and cowardly terrorist attack at Parsons Green.
"As the prime minister has said, this is our highest level of security preparedness.
"I utterly condemn the hideous individuals who target innocent people and attempt to use terror to harm us and destroy our way of life."
No details have yet emerged about the person or persons responsible for the attack, but investigations are ongoing.
Islamic State says a "detachment" of the terror group was behind the attack during Friday morning's rush hour.
