British police have arrested an 18-year-old man in the port of Dover and raided a property in a small town outside London as they hunted for whoever planted a bomb on a commuter train that injured 30 people a day earlier.
Prime Minister Theresa May put Britain on the highest security level of "critical" late on Friday, meaning another attack may be imminent, and deployed soldiers and armed police to strategic sites.
In what police called a "significant" development, officers arrested a man in Dover just before 8am on Saturday, from where passenger ferries sail to France. Five hours later they raided a property in Sunbury, a commuter town southwest of London, and evacuated nearby premises as a precaution.
Pictures on social media showed police in a nondescript residential street in Sunbury, in the county of Surrey.
"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, speaking before details of the Sunbury raid was confirmed. "For strong investigative reasons we will not give any more details on the man we arrested at this stage."
In what was the fifth major terrorism attack in Britain this year, the homemade 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.
According to media reports, the bomb was attached to a timer, unlike other 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 reopened by Saturday morning.
Armed police patrolled the streets of London near government departments in Westminster and were expected to guard Premier League soccer grounds hosting matches on Saturday, including the national stadium at Wembley.
Cressida Dick, Britain's top police officer, sought to reassure the public and tourists as she joined colleagues patrolling the entertainment and cultural district on the south bank of the Thames.
"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."
The last time Britain was put on "critical" alert was after a suicide bomber killed 22 people at a concert by the American singer Ariana Grande in Manchester in May.