Four thousand people are taking part in the biggest test run so far for the London Olympic games.
The nationwide three-day test starting Tuesday is going to involve Games organizers, the British government, emergency services, health providers and transport operators.
The tests aim to mimic August 4-6, the eighth, ninth and 10th days of the London Olympics, where 26 sports will be taking place across 14 venues. Those sites include beach volleyball at the Horse Guards Parade, the triathlon in Hyde Park and tennis at Wimbledon.
Potential incidents will be thrown up to see how everyone involved reacts and test the procedures used to deal with the problems.
This is the third and final command exercise before the 2012 games begin on July 27.
"Obviously nothing can fully prepare us for the reality of hosting the world's biggest sporting event," Culture Secretary Jeremy Hunt said in a statement. "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."
The Olympics run from July 27-August 12, while the Paralympics take place from August 29 to September 9.