London - British authorities were searching for suspects on Saturday in the London subway blast that injured more than two dozen people as hundreds of soldiers were being deployed across the country.British police arrested an 18-year-old man on Saturday in connection to their investigation into the bombing of a packed London Underground train."The 18-year-old man was arrested by Kent Police in the port area of Dover this morning, Saturday, 16 September, under section 41 of the Terrorism Act," police said in a statement describing the arrest as "significant".Authorities increased the terrorism threat level to "critical" late Friday, after a bomb partially exploded during the morning rush hour, meaning a government task force that includes the security services believes another attack may be imminent.Prime Minister Theresa May said raising the threat level to its highest point was a "proportionate and sensible step." Police called on the public to be vigilant.The soldiers will add to the police presence Saturday at public places to deter attacks after the Friday morning rush-hour blast on a District Line train. No arrests have been made. The explosion and an ensuing stampede at Parsons Green station injured 29 people.None of the injuries, some of them burns, were believed to be life-threatening.The bomb went off around 08:20 as the train, carrying commuters from the suburbs, including many school children, was at Parsons Green station in the southwest of the city.The station was reopened on Saturday, officials said, restoring some normalcy to London's transport network after a day of severe disruption.The bomb was put into a bucket and concealed in a shopping bag. 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.The Islamic State group claimed responsibility for the attack, which it said was carried out by an affiliated unit.Britain has endured four other attacks this year, which have killed a total of 36 people. The other attacks in London, near Parliament, on London Bridge and near a mosque in Finsbury Park in north London, used vehicles and knives.In addition, a suicide bomber struck a packed concert hall in Manchester in northern England, killing 22 people. That attack in May also briefly caused the threat level to be set at "critical."