A second man was arrested in the U.K. by detectives from the Metropolitan Police's counter-terrorism command investigating the terrorist attack at the Parsons Green underground station.
The 21-year-old man was arrested in Hounslow, a borough in west London, late on Saturday local time, under section 41 of the Terrorism Act, the police said. He was taken to a south London police station and remains in custody.
Section 41 gives police the power to arrest and detain someone for up to 48 hours on suspicion of being a terrorist, with the potential for the period to be extended to as many as 28 days if a judge agrees.
Reuters reported that Hounslow was about four miles from Sunbury, where police raided a building on Saturday as part of the investigation.
The first arrest earlier on Saturday was of an 18-year-old man, who was detained by Kent Police in the port area of Dover and who remains in custody.
Authorities are treating the bombing of the underground train station as a terrorist attack.
Police did not identify the man.
Late on Friday, U.K. Prime Minister Theresa may put her country on the highest security level, "critical," which means authorities believe another attack could be imminent, Reuters reported.
At least 29 people were injured in Friday's attack, which was carried out with an improvised explosive device that exploded on a crowded commuter train at London's Parsons Green Underground Station during morning rush hour. No one was killed in the incident.
The Metropolitan Police said it was an improvised explosive device and most of those injured had "flash burns."
Police have collected images and videos through a website, the UK Police Image Appeal.
Police asked that anyone with information on the attack call the Anti-Terrorist Hotline at 0800 789 321.
—CNBC's Ted Kemp contributed to this article.