A large-scale exercise to test the military contribution to the Olympics security effort has been taking place.
Tests of air defence missile systems at six sites across London using dummy armaments have begun.
And Typhoon jets, based at RAF Northolt in west London, will take to the skies over south-east England with Lynx, Sea King and Puma helicopters.
Exercise Olympic Guardian, which goes on until 10 May, will also see HMS Ocean sail to Greenwich in the capital.
Military chiefs have warned residents of an increase in loud air activity.
The exercises are testing how RAF personnel, soldiers and sailors will intercept and communicate with aircraft breaching restricted airspace during the London Games, which start in July.
The Ministry of Defence said the operations were building on the air force's existing defence of UK airspace.
Under the Air Security Plan, 30-miles (48km) of airspace surrounding the Olympic Park would become a restricted flying zone.
On the ground, the RAF will provide mobile ground radar systems, while the Army deploys air observers and high-velocity missiles.
The observers would be placed at 14 sites to spot potential air threats using binoculars with thermal imaging detecting systems.
Helicopters with Royal Air Force regiment snipers could also be used to intercept aircraft that entered the restricted airspace without permission, the RAF said.
The arrival of the Typhoon jets at RAF Northolt marked the first time fighter planes have been stationed there since World War II.
The operations also include the deployment of HMS Bulwark and other ships to Weymouth Bay and Portland Harbour.
Defence Secretary Phillip Hammond said: "Whilst there is no specific threat to the Games, we have to be ready to assist in delivering a safe and secure Olympics for all to enjoy."
He said the Typhoon operation at RAF Northolt underlined the "commitment of the Ministry of Defence and our armed forces to keeping the public safe at a time when the world will be watching us".
But the Stop the War Coalition has criticised the move as "unacceptable", arguing that heavy military activity in the capital will cause unnecessary fear.
Lindsey German of the campaign group said: "Far from safeguarding Londoners as they go about their daily lives, they will bring a real fear of explosions and the prospect of these places becoming a target for terrorist attack."
The surface-to-air missiles, which officials said would be protected by armed police, were unveiled at Blackheath Army Cadet Centre in south London.
Military chiefs said two locations have been identified as potential sites for the high velocity missiles if its Air Security Plan wins government approval.
The Lexington Building in the Bow Quarter apartment complex in Bow, Tower Hamlets, and the Fred Wigg Tower in Waltham Forest both have clear views of the Olympic Park in Stratford.
The longer-range Rapier missiles would be positioned on Blackheath Common and in Oxleas Wood, both in south-east London, and at William Girling Reservoir Chain in Enfield and Barn Hill at Netherhouse Farm in Epping Forest, both in north London.
The announcement of the potential locations comes as a resident of the Bow Quarter posted a video of what he claims are missiles left in the complex.
Journalist Brian Whelan is challenging the management company after residents were told a missile system could be put on a water tower.
Mr Whelan's video contains footage of what he describes as "unguarded military rockets" at the foot of the tower. "There is nobody around this military equipment here. Crates, clearly full of missiles and not a person in sight," his voiceover says. Soldiers can be seen walking into shot at the end of the clip.
However, the Ministry of Defence said no live missiles or ordnance were being used in the testing of ground-based air defence systems in London this week.
"The GBAD systems are fitted with practice, dummy missiles which pose absolutely no danger to the public," a spokesman said. "The video in question, taken by an individual who vocally opposes their potential deployment, clearly shows military personnel on the scene."
Air Vice-Marshal Stuart Atha, air component commander for Olympics air security, said the training exercise was "essential" and in line with "preparations for most Olympics" in recent years.
Some 13,500 military personnel are being deployed during the Olympics.
Sites identified as potential locations for Rapier and high-velocity missile systems