Elite police and military teams joined forces today to give a first glimpse of Britain's biggest peacetime security operation.
Scotland Yard and the Royal Marines took to the River Thames in a show of strength against terrorists targeting the Olympics.
Around 100 marines and 50 officers rehearsed a string of high-speed drills amid fears attackers could use the waterways to launch onslaughts on London landmarks.
About 50 marine police officers in rigid inflatables and fast response boats were joined by up to 100 military personnel and a Lynx Navy helicopter for the familiarisation exercises.
Assistant Commissioner Chris Allison, head of the force's security operation for the Games, said a Mumbai-style terror attack was one of a host of potential threats to the games.
Speaking on the shores of the Thames, he said: "There is no specific threat from the river but we would be failing in our duty to ignore it at games time.
"What you have seen today is the sort of things we can do."
Despite a heavy military presence on show today, Mr Allison insisted the sporting event would remain a "blue games".
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.
"If we need the military support, it is there," he added.
"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."
Major Paul Stitson, of 539 Assault Squadron, said police and the military had combined efforts seamlessly.
"We have been developing our local knowledge and assessing threats," he said. "We always prepare for the unknown."
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."
The Government has stressed that operations in Afghanistan and elsewhere will not be affected by the security plan, which is 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.
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