An 18-year-old man has been arrested on suspicion of a terror offence in connection with Friday’s attack on a London Tube train.
The man was held in Dover on Saturday and taken to a Kent police station – he will be moved to south London later.
The arrest is “significant” but the terror threat level remains at “critical”, Metropolitan Police Deputy Assistant Commissioner Neil Basu said.
Thirty people were injured after the explosion on a train at Parsons Green.
Met Police Commissioner Cressida Dick described the attack as “cowardly and indiscriminate”.
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.”
Home Secretary Amber Rudd chaired a meeting of the government’s Cobra emergency committee and it concluded the UK terrorist threat level should remain at “critical”.
It had been raised to that level – which is the highest and means an attack may be imminent – on Friday evening.
The public should remain vigilant, Mr Basu said, as the force was not changing its “protective security measures” and extra armed officers were still being deployed.
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.
Analysis: A ‘critical’ change
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.
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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Alexis Sanchez scored a stunning goal as Arsenal came from behind to beat Cologne in a Europa League match delayed for an hour following crowd trouble.
Chilean Sanchez, who nearly left the club to join Manchester City on transfer deadline day, picked the ball up outside the area and curled a shot past keeper Timo Horn.
The game did not begin until 21:05 BST as thousands of visiting supporters arrived at the ground without tickets and then clashed with stewards inside the Emirates Stadium.
When the match did get under way, Cologne took the lead in spectacular fashion as Jhon Cordoba lobbed fellow Colombian David Ospina from 40 yards.
Striker Olivier Giroud planted a header wide of goal from six yards for the much-changed home side, who were booed at the half-time whistle.
But substitute Sead Kolasinac equalised with a thumping volley before Sanchez struck, and Hector Bellerin then added the third from close range, as midfielder Jack Wilshere made his first appearance for the club since August 2016.
In Group H’s other game, Red Star Belgrade drew 1-1 against BATE Borisov.
Relive Arsenal’s victory over Cologne
The importance of Alexis
Forward Sanchez scored 30 goals in all competitions for Arsenal last season as they won the FA Cup.
But in August, he almost joined former boss Pep Guardiola at Manchester City for £60m, but the Gunners pulled out of the deal after failing to find a replacement.
Sanchez made his second start of the season in an impressive showing, capping his performance with a sublime effort in the second half – his first goal of the season against the Bundesliga’s bottom side.
He could have scored another two, but struck a free-kick straight at Horn and shanked wide from inside the area.
Forgotten man Wilshere, who spent last season on loan at Bournemouth before suffering a broken leg, came on for Alex Iwobi on 68 minutes, and the England international’s clever dummy was instrumental in the lead-up to Bellerin’s goal.
20,000 does not fit into 2,900
Cologne fans had been given a ticket allocation of 2,900 but 20,000 fans travelled over from Germany
Read more: Arsenal match delay ‘in the interest of crowd safety’
Cologne, like Arsenal, finished fifth in their domestic league last season, achieving European football for the first time in 25 years.
The Bundesliga club’s return to European competition, though, was one tainted by controversy.
There were indications on Thursday afternoon that problems may occur, with more than 20,000 fans arriving from Germany, despite the visitors receiving an allocation of 2,900 tickets.
Videos emerged on social media of the German side’s support briefly bringing parts of central London to a standstill as they threw bottles and let off flares while making their way to the ground.
The kick-off was then put back, after which skirmishes between fans and stewards took place, while many had entered the ground into the home end, climbing barriers to get into the away section.
A number of Arsenal fans inside texted BBC Football, with one supporter reporting that they felt “intimidated by the horrible, tense atmosphere”, another that they were “ashamed of Arsenal for not seeing the warning signs” and a third describing it as “the worst feeling at football in 40 years of watching”.
It remains to be seen whether Uefa charges are brought against one or both sides.