Elite British security teams take to the River Thames today as part of a massive security rehearsal to foil terror disaster at the Olympics.
Scotland Yard and the Royal Marines are joining forces for a string of "familiarisation exercises" to avoid catastrophe in London this summer.
The Metropolitan Police's Marine Policing Unit and army members will provide a rare glimpse of Britain's top secret security plan as they take to the water.
The drills "are designed to make sure that everyone has the same level of knowledge about how the river operates and how our tactics work together," police said.
Assistant Commissioner Chris Allison, head of the force's security operation for the Games, said: "All of our planning is designed to mitigate against potential risks during the summer of 2012, and this is an example of where we will be using specialist military capability to support us.
"This exercise is not in response to any specific threat, but is part of our planning to pre-deploy certain specialist assets to bolster our operation.
"This will be a summer like no other in London. The Thames runs through the very heart of our capital and will be a popular place for people who want to be part of the Olympic spirit. This is all part of our planning to ensure this summer's events take place safely and securely."
About 44 marine police officers in rigid inflatables and fast response boats will be joined by up to 94 military personnel and a Lynx Navy helicopter.
Crime and security minister James Brokenshire said: "This exercise forms part of the comprehensive testing and exercise programme that is crucial in securing the Games.
"We are leaving nothing to chance to ensure we deliver a safe and secure Games that London, the UK and the world can enjoy."
Typhoon jets and HMS Ocean, the largest ship in the Royal Navy's fleet, will eventually be deployed to protect the London 2012 Games along with up to 13,500 military personnel.
The Government has stressed that operations in Afghanistan and elsewhere would not be affected by the security plan costing hundreds of millions of pounds.
Typhoon jets will be stationed temporarily at RAF Northolt, and Puma and Lynx helicopters will operate from HMS Ocean.