Britain's security services have not given details of potential terror activity so far this year, but in December the head of security service MI5, Andrew Parker told politicians that during 2017 a total of nine terrorist attacks had been prevented in Britain. Five attacks, including the Manchester bombing and others in London, had got through, four related to Islamist terrorism.

Later Tuesday, more than 3,000 singers from local choirs were scheduled to join forces and share the spirit of solidarity at a "Manchester Together -- With One Voice" event, including a choir of people who were at the arena when the blast struck.

At 10.31 p.m. local time, church and city hall bells will ring out from buildings across the city center to mark the moment of the attack. Meanwhile, thousands of people have left messages on 22 "Trees of Hope" close to the scene of the attack.