An improvised explosive device (IED) exploded on a packed underground train at a London station on Friday, injuring at least 29 people. It was the fifth terrorist attack in the UK this year.
A second man has been arrested by police investigating Friday's bomb attack on a London underground train, police said.
The 21-year-old man was arrested in the west London suburb of Hounslow just before midnight on Saturday, the Metropolitan Police force said in a statement.
He was detained under Britain's Terrorism Act and taken to a south London police station, the force added on Sunday.
An improvised explosive device (IED) exploded on a packed underground train at a London station on Friday, injuring at least 29 people. Prime Minister Theresa May said it was "clearly intended to cause significant harm".
The attack was claimed by the Islamic State.
Several people suffered facial burns, some were reportedly even trampled in the rush to escape. But none of the injuries were life-threatening.
Pictures of a white bucket on fire inside a supermarket bag, with wires trailing on to the carriage floor of the train, went viral on social media.
This was the fifth terrorist attack in the UK this year. But it was the only one in 2017 in which nobody died.