The UK's terror threat level has been reduced from critical to severe, Theresa May has said.
This means the likelihood of an attack is highly likely rather than imminent.
Operation Temperer, which allows the military to be deployed to key sites, will continue until the end of the Bank Holiday, the Prime Minister said.
Speaking after a meeting of the Government's Cobra emergency committee, Mrs May said the decision had been taken after "a significant amount of police activity" over the last 24 hours.
She said: "The public should be clear about what this means - a threat level of severe means an attack is highly likely. The country should remain vigilant."
New CCTV of the Manchester bomber Salman Abedi has been released by police.
The younger brother of the Manchester bomber has said Salman Abedi was radicalised online in Britain.