The appearance of fighter jets in the skies over London will mark the start of an extraordinary Olympic test event.
Operation Olympic Guardian is an eight-day military exercise to train pilots, snipers and missile operators.
The most obvious evidence of military activity will be the low-flying RAF Typhoon fighters armed with air to air missiles.
Their noisy presence will be felt across London and the Home Counties with residents living close to RAF Northolt in West London especially affected.
The RAF station, more used to handling VIP flights and business jets, will become the forward operating base for the Typhoons and Royal Navy Sea King helicopters.
The Royal Navy helicopter carrier HMS Ocean will sail up the Thames to Greenwich on Friday to act as "mother ship" for sniper-carrying Lynx helicopters and Royal Marine patrol boats.
Surface-to-air missile batteries will be set up on two residential blocks in East London as well as four other sites around the capital.