A second man has been arrested over Friday's bomb attack on a London Underground train that injured at least 30 people, police said.
The 21-year-old man was arrested in the west London suburb of Hounslow just before midnight on Saturday, the Metropolitan Police said in a statement.
He was detained under Britain's Terrorism Act and taken to a south London police station, police said.
Officials have said they are not sure how many people were involved in the bombing, which prompted the government to raise country's terrorism warning to critical, the highest level.
In the wake of the attack, there was no sign of panic among Londoners and the weekend life of the city continued undeterred by the raised threat level.
Officials said the bomb was intended to do grave harm to commuters. Analysts said the injuries would have been far worse had the entire device exploded.