British police said Sunday they had made a second arrest in connection with the bombing of a London Underground train, as their probe into the terror attack widened.

The 21-year-old man, who has not been named, was arrested 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, and said they were hunting for more suspects.

The bomb detonated in a packed train carriage Friday morning with a large explosion followed by what an eyewitness described as a ‘fireball’.

It was Britain's fifth terror attack in six months -- a series that has claimed 35 lives.

The Islamic State group claimed responsibility.

The first arrest on Saturday took place at the Dover ferry terminal -- a main link to Europe. A ‘number of items’ were recovered during the operation and the teenager is now in custody in London, officers said.

Police had earlier raided a home in Sunbury, a town west of London. Local residents quoted in British media said the owners of the house were elderly foster parents.

- Terror threat 'critical' -

Britain's terror threat was raised on Friday to ‘critical’, indicating that another attack is feared, and soldiers have been deployed to guard key points to free up police for the investigation.

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

Home Secretary Amber Rudd said 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.

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.’

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

The improvised device at Parsons Green, a quiet and well-off residential district, failed to detonate fully.

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

- 'Fireball' -

Twitter user @Rrigs posted pictures of a white bucket smouldering on the train and described how a ‘fireball flew down carriage and we just jumped out open door’.

The bucket, which was inside a frozen food bag, looked like the type used by builders and there appeared to be cables coming from it.

Louis Hather, 21, had been travelling to work and was three carriages down from where the explosion took place.

‘I could smell the burning. Like when you burn plastic,’ he told AFP.

He was trampled on as panicking passengers stampeded out of the station and his leg was badly cut and bruised.

The bomb's remnants were examined by forensic scientists but no further details were released.

Several victims were taken to hospital, though health authorities said none were in a serious life-threatening condition.