British police arrested an 18-year-old man in the southern port of Dover on Saturday in a "significant" development in the hunt for the culprits behind a London commuter train bombing that injured 30 people a day earlier.
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Prime Minister Theresa May put Britain on the highest security level of "critical" late on Friday, meaning an attack may be imminent, and deployed soldiers and armed police to secure strategic sites and hunt down the perpetrators.
In the fifth major terrorism attack in Britain this year, the home-made bomb shot flames through a packed commuter train during the Friday morning rush hour in west London but apparently failed to detonate fully.
The militant group Islamic State claimed responsibility.
"We have made a significant arrest in our investigation this morning," said Neil Basu, Senior National Co-ordinator for Counter Terrorism Policing.
"This arrest will lead to more activity from our officers," he said, suggesting there could be more arrests and house raids to come. "For strong investigative reasons we will not give any more details on the man we arrested at this stage."
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The arrest was made in the port area of Dover, where passenger ferries sail to France.
According to media reports, the bomb was attached to a timer unlike recent blasts which have typically been suicide bombs.
Pictures showed a slightly charred white plastic bucket with wires coming out of the top in a supermarket shopping bag on the floor of a train carriage.
The Parsons Green station where the attack took place had reopened by Saturday morning.
Armed police patrolled the streets of London near government departments in Westminster and were expected to guard the Premier League soccer grounds hosting matches on Saturday, including the national stadium of Wembley.
In the entertainment and cultural district on the south bank of the Thames, Cressida Dick, Britain's top police officer, sought to reassure the public and tourists as she joined colleagues patrolling the area.
"Yesterday we saw a cowardly and indiscriminate attack which could have resulted in many lives being lost," she said. "London has not stopped after other terrible attacks and it will not stop after this one."
CRITICAL THREAT LEVEL
The last time Britain was put on "critical" alert was after a suicide bomber killed 22 people, including children, at an Ariana Grande concert in Manchester in May.
The threat level remained at the highest setting for four days while officers raced to establish if the man had worked alone or with the help of others. Prior to that it had not been triggered since 2007.
Prime Minister May said the public should not be alarmed by armed officers on the streets, a rare sight in Britain. "This is a proportionate and sensible step which will provide extra reassurance and protection while the investigation progresses," she said in a televised statement late on Friday.
The bomb struck as passengers were travelling to the centre of the British capital. Some suffered burns and others were injured in a stampede to escape from the station, one of the above-ground stops on the underground network. Health officials said none was thought to be in a serious condition.
With Britain on high alert after a spate of attacks this summer, witnesses recalled their horror.
"I was on the second carriage from the back. I just heard a kind of 'whoosh'. I looked up and saw the whole carriage engulfed in flames making its way towards me," Ola Fayankinnu, who was on the train, told Reuters.
"There were phones, hats, bags all over the place and when I looked back I saw a bag with flames."
The Islamic State militant group have claimed other attacks in Britain this year, including two in London and the pop concert in Manchester.
It was not immediately possible to verify the claim about Parsons Green, for which Islamic State's news agency Amaq offered no evidence.
Western intelligence officials have questioned similar claims in the past, saying that while Islamic State's jihadist ideology may have inspired some attackers, there is scant evidence that it has orchestrated attacks. (Reporting by Kate Holton; editing by David Clarke)