London, Sep 17 (IANS): A second man was arrested over the terror attack on a London Tube train that left 30 people injured and resulted in an increase in the country's terror threat level from "severe" to "critical", police said on Sunday.

According to the Metropolitan Police, the 21-year-old man arrested in Hounslow, west London, on Saturday night, reports the BBC.

"He was arrested under the Terrorism Act and taken to a police station where he remains in custody," the Met police said.

Earlier on Saturday, an 18-year-old man was arrested the Port of Dover over the Friday morning attack, sending a ball of fire along a carriage of the eastbound District Line train from Wimbledon at the Parsons Green station.

Police are continuing to search a house in Sunbury-on-Thames in Surrey. The house belongs to an elderly couple known for fostering hundreds of children, including refugees.

The Islamic State terror group has claimed responsibility for the attack, the BBC reported.

The explosion occurred through a homemade explosive artefact, inside a white bucket, which failed to fully detonate.

Less than 24 hours later, the suburban Tube station was back in use by travellers.

Prime Minister Theresa May raised the terror status to its highest level, which means another attack is considered imminent.