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 locations such as nuclear power plants.

On Saturday morning officers in Dover, a ferry port on the southeast coast from which passenger ships sail to France, arrested a man and then partially evacuated the area, recovering a number of items, the police said without elaborating.

Hours later, police raided a house in a commuter town southwest of London, and evacuated nearby premises as a precaution. Police officers in forensic suits were seen entering a modest house in a suburban street in Sunbury, around 11 miles (18 km) from Parsons Green where the bomb exploded.

The police said they were keeping an open mind as to whether more than one person was involved in the Friday morning attack on a packed train in west London.

"We are still pursuing numerous lines of enquiry, and at great pace," Senior National Co-ordinator for Counter Terrorism Policing Neil Basu told reporters.

"We're open-minded. If there are other people responsible it's our job to find them and that is part of the reason why we are remaining at 'critical' at the moment."

Officials said the attack could have been deadly. The home-made bomb shot flames through a packed train carriage but apparently failed to detonate fully.

According to media reports, the device 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.

"This is a very significant arrest," said interior minister Amber Rudd of the 18-year-old. "The police have made very good progress but the operation is ongoing," she said.

"There is no doubt that this was a serious IED (improvised explosive device) and it was good fortune that it did so little damage."

Islamic State claimed responsibility. The group has claimed other attacks in Britain this year, including two in London and one at a concert by American singer Ariana Grande in Manchester, but intelligence officials say there is scant evidence that it had orchestrated them.

CRITICAL THREAT LEVEL

Armed police patrolled the streets of London near government departments in Westminster and guarded 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 as she joined colleagues patrolling the entertainment 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 the Ariana Grande concert in May.

The threat level remained at the highest setting for four days while police 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 traveling to the center 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.

"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 told Reuters.

"There were phones, hats, bags all over the place and when I looked back I saw a bag with flames."

(Reporting by Kate Holton; Editing by David Clarke and Robin Pomeroy)