Prime Minister Theresa May has raised Britain's threat level from severe to critical - the highest level - after last night's attack in Manchester.
In a statement at Downing Street, May said she didn't wish to alarm the British public, but had taken the decision in light of the investigation into the terror attack at Manchester Arena which left 22 people dead.
The 'critical' level indicates the authorities are preparing for an imminent terror attack. It's only the third time in British history that the highest threat level has been invoked.
May also said troops would be deployed at major events and locations to replace armed police, although it is not presently clear how widespread their deployment will be.
Tributes have been pouring in all day to the victims of last night's attack, which took place at the end of a concert by US singer Ariana Grande. Many of the victims were children.
The terrorist suspected of perpetrating the attack has been named as 22-year-old Salman Abedi, while a 23-year-old man has also been arrested. Raids have taken place in the south Manchester suburbs of Whalley Range and Fallowfield, where Abedi was believed to be living.