May 27 (UPI) -- Britain's terror threat level was lowered from "critical" to "severe," Prime Minister Theresa May said Saturday after meeting with security leaders.
"The public should be clear about what this means," May said. "A threat level of 'severe' means an attack is highly likely -- the country should remain vigilant."
The Joint Terrorism Analysis Center had increased the level to "critical" -- meaning an attack is imminent -- after Monday's suicide bombing that killed 22 people at an Ariana Grande concert in Manchester.
Troops to bolster police numbers will be gradually withdrawn from the streets starting Monday, May said.
The change came as counter-terror officers evacuated a street in Manchester on Saturday morning. One witness reported seeing a bomb-disposal van parked at an intersection. An address in the area was being searched as detectives sought members of a suspected terror cell behind named suicide bomber Salman Abedi.
Abedi was a British national of Libyan descent.
The lowered terror threat level hasn't stopped British police from stepping up enforcement. The country is taking the unusual step of arming police for patrols in Manchester, London and other locations this weekend during spring bank holiday, the country's first holiday since the bombing. Some events have been canceled, including the Chelsea Football Club parade Sunday to honor its Premier League victory, and others are planned with additional security.
"All of this is designed to make the policing approach unpredictable and to make London as hostile an environment as possible to terrorists," said Chief Superintendent Jon Williams of London's Metropolitan Police Service.
Early Saturday, two more men were arrested and taken into custody after officers carried out a controlled explosion to enter a home in central Manchester, police said in a statement. So far, 13 people have been arrested in the investigation, with 11 still in custody.
"The understanding of [the plot] is growing day by day," Britain's top counterterror officer, Assistant Commissioner Mark Rowley, said Friday. "We've got our hands around some of the key players. We still have more to do."