'Sonic gun' deployed for Olympics

The equipment, which can project a piercing sound over hundreds of metres causing physical pain, has been used during protests at the G20 summit in Pittsburgh in 2009 and against pirates operating off the Somali coast.

The Ministry of Defence said it expected to use it primarily in loudspeaker mode to communicate with boats it wants to stop on the River Thames.

Defence chiefs have already caused controversy by announcing plans to put surface-to-air missiles on the top of residential buildings near the Olympics site in east London.

Fighter jets roared over the capital and helicopters were seen hovering over the Houses of Parliament this month during a nine-day military exercise to prepare for Britain's biggest peacetime security operation.

The LRAD (Long Range Acoustic Device), made by U.S. company LRAD Corporation, can be mounted on the side of a ship or on the top of a vehicle. Some versions are roughly the same size and shape as a dustbin lid.

It can generate noises up to around 150 decibels, similar to a gunshot, and has a maximum range of 3 km or 1.8 miles.

The device can also emit a warning alarm that sounds like a police siren and "potentially prevents the use of harmful or deadly force", LRAD says on its website.

Police and military planners say they are preparing for a range of security threats at the Olympics including protesters trying to disrupt events and attacks using hijacked airliners.

Around 13,500 soldiers will help the police during the Games, which run from July 27 to August 12.