Image copyright Anar Allidina Dale Image caption Residents in the Cavendish Road area of Sunbury, Surrey, have been told to leave their homes
Armed police have evacuated and are searching a house in Sunbury, Surrey, in connection with Friday's attack on a London Tube train.
It follows the arrest of an 18-year-old man on suspicion of a terror offence.
The arrest is "significant" but the terror threat level remains at "critical", Home Secretary Amber Rudd said.
The man was held in Dover on Saturday and taken to a Kent police station - he will be moved to south London later.
Thirty people were injured after the explosion on a train at Parsons Green.
In Sunbury, residents near to the address have been told to leave their homes and cordons are in place at a 100-metre radius to the property.
Ms Rudd chaired a meeting of the government's Cobra emergency committee and it concluded the UK terrorist threat level should remain at "critical".
After the meeting, she said it was "much too early" to say whether those behind the attack were known to authorities, adding: "The operation is ongoing".
Metropolitan Police Commissioner Cressida Dick described the attack as "cowardly and indiscriminate".
Media playback is unsupported on your device Media caption Home Secretary Amber Rudd says it was "good fortune" that the bomb at Parsons Green Tube station did "so little damage".
Ms Dick, who travelled by Tube to Waterloo station before joining Met officers patrolling the South Bank, said: "London has not stopped after other terrible attacks and it will not stop after this one."
It had been raised to that level - which is the highest and means an attack may be imminent - on Friday evening.
Met Police Deputy Assistant Commissioner Neil Basu said the public should remain vigilant, adding that the force was not changing its "protective security measures" and extra armed officers were still being deployed.
Image copyright PA Image caption Met Police Commissioner Cressida Dick joined police patrolling London's South Bank
Operation Temperer, which involves the use of the military and was put in place after the threat level was raised, is being stepped up gradually.
Home affairs correspondent Danny Shaw said it is focused on London and the south-east of England and is being described as "light-touch", and not on the same scale as what was seen after the Manchester Arena attack.
Mayor of London Sadiq Khan said "significant" police activity would continue over the weekend and thanked police, adding: "They are there to keep us safe".
More armed police officers will be present at London Underground stations, as well as at stations across England, Scotland and Wales, British Transport Police Assistant Chief Constable Robin Smith told the BBC.
Most people caught in the blast were treated for minor injuries and have been released, NHS England said, but three people remain in Chelsea and Westminster hospital in central London.
By BBC security correspondent Frank Gardner
This is the fourth time the UK national terror threat level has been raised to "critical" since the system was made public in 2006.
The last time was in May this year following the Manchester Arena bombing, when it was wrongly thought that the bomb-maker was still at large and could strike again.
In the case of Parsons Green it is perhaps surprising that it took the government so long - over 12 hours - to raise the threat level to critical, under advice from the Joint Terrorism Analysis Centre, when it was obvious the perpetrator had neither died nor been caught.
Each time the level has gone to critical, it has only stayed at this highest level of alertness for three to four days - this is partly as it involves an unsustainably high tempo for the police, intelligence and security services.
Extra patrols are mounted on the streets of London, covert surveillance is stepped up and troops are deployed to free up police officers to focus on the main effort: catching the bomber before he can plant another device.
But the very fact that yesterday's attack took place with no warning shows this system is only a broad guide to the threat and simply reflects the latest assessment.
The BBC understands CCTV images have been obtained of the person suspected of planting the explosive device.
The Islamic State group has said it was behind the bomb, which detonated at 08:20 BST on Friday.
It is understood the device had a timer, but the BBC's security correspondent Frank Gardner said the bomb appeared not to have gone off properly.
Image copyright PA Image caption Police have spoken to 45 witnesses so far and received 77 images and videos from the public
Had it worked as intended, it would have killed everyone around it and maimed everyone in the train carriage for life, he said.
The Met's Assistant Commissioner Mark Rowley said it was "very routine" for the Islamic State group to claim the attack, whether in contact with those involved or not.
Police have spoken to 45 witnesses so far and have received 77 images and videos from the public.
They urge anyone with information to get in touch and to upload pictures and video to the website www.ukpoliceimageappeal.co.uk or to call the Anti-Terrorist Hotline on 0800 789 321.
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Skagawa99 on September 16th, 2017 at 14:00 UTC »
Waiting for the inevitable: 'He was known to the authorities but was not considered an active threat..'
extendedlead on September 16th, 2017 at 09:53 UTC »
UPDATES AS IT HAPPENS:
This is just me speculating, but since he was caught in the port area of Dover, it seems like he was trying to leave for Europe. /u/Adarale says the same below.
blackhawk1012 on September 16th, 2017 at 09:48 UTC »
With all the CCTV it was surely a matter of time