Four thousand people are taking part in the biggest exercise to test plans for the London 2012 Games.
Key backroom staff including Games organisers, the Government, emergency services, councils, health providers and transport operators are part of the nationwide three-day test starting on Tuesday.
It is being staged to mimic August 4-6, the eighth, ninth and 10th days of the Olympics where 26 sports will be taking place across 14 venues, including beach volleyball at Horse Guards Parade, the triathlon in Hyde Park and tennis at Wimbledon.
Potential incidents will be thrown up to see how everyone involved reacts, the procedures used to deal with the problems and to inform the public what is going on.
This is the third and final command post exercise before the Games begin on July 27.
Culture Secretary Jeremy Hunt said: "Obviously nothing can fully prepare us for the reality of hosting the world's biggest sporting event, but we really want to make sure we do all we can to ensure we deliver an Olympics and Paralympics that are as safe and secure as possible, while also making sure everyone involved has a great time."