Groups of soldiers have been seen outside of Downing Street and Parliament after the terror threat level was raised to critical.
Armed troops are being deployed to guard "key locations" such as Buckingham Palace and Number 10 as part of Operation Temperer.
Nearly 1,000 members of the military are expected to be deployed.
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.