A second man has been arrested in connection with the bombing of a London Underground train, police said on Sunday after announcing that they were hunting for more suspects.

The 21-year-old man was detained late Saturday in Hounslow, on the western rim of the capital, a statement said.

Officers had earlier arrested an 18-year-old man over Friday's attack at Parsons Green station, which injured 30 people. The man, who has not been named, was arrested on Saturday on suspicion of committing, preparing or instigating an act of terrorism.

"We're keeping an open mind around whether more than one person is responsible for the attack," Neil Basu of London's Metropolitan Police told reporters.

Dover's ferry terminal was evacuated during the arrest and "a number of items" were recovered, while the teenager was held in custody in London.

The bomb detonated in a packed train carriage on Friday morning with a large explosion followed by what eyewitnesses described as a "fireball". It was Britain's fifth terror attack in six months.

The Islamic State group claimed responsibility.

Britain's threat level has been raised to "critical", indicating that another attack is feared, and soldiers have been deployed to guard key points to free up police for the investigation.

Home Secretary Amber Rudd said on Saturday that police had made "very good progress" in their enquiries, while appearing to dispute claims by US President Donald Trump that a "loser terrorist" behind the attack was known to Scotland Yard.

"It's much too early to say that," Rudd said in a televised interview. "At the moment, we have one arrest and we have an ongoing operation."

Trump's claims, made Friday on Twitter, had already garnered a terse rebuke from Prime Minister Theresa May, who said: "I never think it's helpful for anybody to speculate on what is an ongoing investigation."

The improvised explosive device in a train at Parsons Green station, a quiet and well-off residential district, failed to detonate fully, according to media reports.

But the blast inflicted flash burns on passengers, and prompted dozens of others to flee in panic.

May announced on Friday that 1,000 troops would be deployed to take on the responsibility for guarding key sites, including nuclear facilities.

The critical alert terror warning was last used after the deadly suicide attack at Manchester Arena, also claimed by IS, in May.

The UK government said on Sunday that it wanted to push for a new security treaty with the EU, which would allow for further cooperation in the fight against crime and terrorism after Brexit.