As crowds gather to watch this year's Olympic opening ceremony in London, highly trained eyes on the ground and in the sky will be keeping tabs on every aircraft over the southeast of Britain.

The first major rehearsal of a joint operation featuring fighter jets, Airborne Warning and Control System (AWACS) planes, helicopters, missile batteries and air-to-air snipers took place two hours north of London on Wednesday, in a plan to supplement Britain's normal airspace security for the games.

This is part of a routine posture that we adopt to protect the UK against airspace intrusions, it's just that because there will be such a tight focus around London and the area around the Olympic Park, we're reconfiguring the assets to be able to respond to threats in a smaller area, Secretary of Defence Philip Hammond said on a visit to Waddington RAF base in eastern England to view the exercise.

Many of the changes to prepare for the Olympics were based on moving forces closer to London to lower response times, and he added that this would add a cost in the few tens of millions of pounds to the overall Olympic budget.

While some aspects may be part of normal operations, the control of such a wide variety of layered air defences by one single team takes special adjustment, the crews involved said.

Usually we don't work with ATC air radar controllers as they're too high to see us, RAF Puma helicopter pilot Lee Cooper told Reuters.

Cooper will be in one of the Puma helicopters carrying air-to-air snipers capable of flying up to and alongside wayward aircraft that may cross into prohibited airspace around the Olympic venues and London.

Keen to avoid incidents such as the errant single engine Cessna that caused a panicked evacuation of the U.S. Capitol building in 2005, all of the elements are focused on early identification and interception of unknown elements, with Typhoon fighters and E-3D radar sentry planes patrolling the skies.

Ground-based surface to air missiles may also be stationed in the centre of London to serve as a last resort, but Hammond said a decision would be taken on whether to ultimately include them or not closer to the Games which open on July 27.

LISTENING DEVICES

The Olympics have seen escalating states of preparedness in the summer games since the September 11 U.S. attacks, with Greece spending roughly $1.2 billion (752.8 million pounds) on security for the 2004 Games including measures such as blimps with listening devices. Beijing stationed surface to air batteries near venues to reassure international visitors in 2008.

I haven't been conscious of any international pressure to beef up Olympic security, Hammond said. Of course our international partners are keen to know how we are going to deploy appropriate security during the Olympics ... and these kinds of exercises are showing the kind of deployments that we will make.

Britain looks set to spend more than a billion pounds on keeping the Games secure, but some countries are still reported to have doubts. A Guardian report in November said the United States was planning to send 1,000 of its own agents to protect American athletes.

I would expect that countries like the United States and Israel to be sending their own agents as it were to provide close protection for their teams, but clearly we will be providing the overall envelope of security for the Olympic Games and the military component you see today is just a part of that, Hammond said.

(Editing by Ed Osmond)