Britain's terrorist threat level has been reduced one level to "severe", Prime Minister Theresa May said Saturday, after a series of arrests linked to last week's bombing of a concert hall in Manchester.

May announced the lowering of the threat level following a so-called Cabinet Office Briefing Room, or COBRA, meeting with security services in central London. The decision follows 11 arrests by police in Greater Manchester and around the country in connection with with deadly attack, which killed 22 people, including and 8-year old schoolgirl, and injured as many as 60 others on May 21. 

"A significant amount of police activity has taken place of the last 24 hours and there are now 11 suspects in custody," May said in a statement. "In light of these developments, JTAC - the joint terrorism analysis centre - has taken the decision to reduce the level from critical to severe."

The alleged perpetrator of the bombing, 22-year old Salman Abedi, was killed in the attack, police said. Both his father and brother, however, have been detailed in the Libyan capital of Tripoli.

May had raised the threat level from "critical" to "severe", the highest of five stages of alert, on Tuesday under a protocol called 'Operation Temperer' that could up to 5,000 troops onto British streets at public events, such as sports competitions, although the military personnel would come under police command.

The threat increase marks only the third time Britain has reached the fifth of five levels, and the first since 2007 when a man drove a Jeep Cherokee loaded with explosives into the Glasgow International Airport in Scotland.

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