General Sir Nick Parker, in charge of the military's Olympics role, said the security exercise would prepare for the possibility of "extreme threats".
What we need to do is make sure we practise against those high-end threats but they are not considered to be likely.
What I'm doing is testing my systems so I'm reassured that, should they become more likely, we can react.
One would want the world to know that we are taking security for the Olympics seriously.
The Royal Navy's largest warship, HMS Ocean, sailed up the Thames today as the military stepped up security preparations for the Olympics.
Details of a vast security operation to defend Britain's air, land and sea during the London Olympic Games became a little clearer today.
A video posted online apparently shows military equipment, being used as part of Olympics security, left unguarded outside London flats.