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As preparations begin for a major Olympics security exercise, here is a round-up of measures planned as part of the Met Police-led operation to keep the London 2012 Olympic Games safe.

The Met and 10 other police forces, as well as thousands of troops and security staff, will all play their part.

Here are the key steps being taken on land, in the water and in the air in the biggest peacetime security operation the UK has seen.

In addition to normal policing levels, there will be 12,500 police officers on Olympic duty in the UK with 9,500 of those working in London, according to the Home Office.

Up to 13,500 military personnel - including 2,100 armed force reservists - will also help to provide security for the Games, which have a security budget of £553m.

That figure includes 5,000 supporting the police in specialist work - including bomb disposal, building search teams and specialist sniffer dogs - and 1,000 providing logistical support.

And up to 7,500 will provide venue security.

The government says these unarmed troops will work alongside civilians "in mixed teams, searching and checking people going into the stadiums, making sure - airline-style - that nothing that shouldn't get in there, gets in".

They will come under the charge of private security contractor G4S which has overall control of Olympic venue security - which covers more than 30 sporting venues and more than 70 "non-competition venues" including car parks and hotels.

The government has said a total security force of 23,700 - which includes the 13,500 military personnel - will take care of venues.

That figures also includes 10,400 staff recruited by G4S as well as a number of volunteers.

The Royal Navy's HMS Bulwark will be stationed off Weymouth and Portland in Dorset - home to sailing and windsurfing events.

It will have a specialist unit of Royal Marines on board, with the capability to deal with terrorist threats and to provide a deterrent against attack.

It will act as the command and control vessel for the security operation surrounding the sailing events.

Dorset Police marine officers can also take to the water to provide security - on jet skis, inflatable boats and smaller craft.

On the River Thames, meanwhile, the Met's marine unit and Royal Marines can operate on patrol boats, inflatables and other craft.

The Royal Navy's largest ship, HMS Ocean, will be moored at Greenwich and used for logistics support, accommodation and as a base for Puma and Lynx helicopters to support airspace security.

The Ministry of Defence (MoD) says it will boost existing numbers of the RAF Typhoon fighter planes that are always on alert to secure British airspace.

A number of the aircraft will be deployed to RAF Northolt in west London.

Royal Navy and Army helicopters will also operate from several sites in and around London.

Surface-to-air missiles could also be deployed, controversially, at six sites across London - including on top of residential flats.

The MoD says "ground-based air defence systems could be deployed as part of a multi-layered air security plan for the Olympics, including fast jets and helicopters, which will protect the skies over London during the Games".

"As part of our ongoing planning, we can confirm site evaluations have taken place."