Surface-to-air missiles could be deployed at six sites across London to prevent any aerial terror attacks on Olympic venues during this summer's Games, the Ministry of Defence has said.
Officials are consulting with residents and housing associations at several locations - including two apartment blocks - in the capital.
The rooftops Lexington House in Bow, Tower Hamlets, and the Fred Wigg Tower in Waltham Forest, both in east London, have been identified as potential sites to host the weapons, capable of shooting down aircraft.
Blackheath Common and Oxleas Wood, both in southeast London, plus William Girling Reservoir in the Lea Valley Reservoir Chain in Enfield, and Barn Hill at Netherhouse Farm in Epping Forest, are also being considered as possible locations.
The move forms part of Olympic Guardian - a major exercise this week involving the testing of security and defence systems by the British army, navy and air force, together with the police.
This includes consultations on the placement of two types of missiles during the Games - the Rapier, and high velocity Starstreak.
Residents and tourists will be able to see typhoon jets and helicopters flying over the city and HMS Ocean venture up the Thames.
Sam Kiley, Sky News defence and security editor, said: "There is going to be a big exercise coming up from May 2, running for nine days.
"They are rehearsing all of the worst-case scenarios. Among the doomsday scenarios is somebody in a light aircraft loaded with a bomb who wants to plough it into the stadium, or a highjacked airliner."
He said the missiles would only be used in the "last line of defence", but the security services were "not leaving anything to chance".
"This is routine. This is exactly what happened at all the other Olympics," he added.
Defence Secretary Philip Hammond said: "Support for the Olympic Games will be an important task for defence in 2012 and this exercise is about pushing our people and our systems to the limit to ensure that we are ready for the challenge.
"The majority of this exercise will be played out in full view of the public and I hope that it will have a secondary effect of reassuring the British people that everything possible is being done to ensure this will be a safe and secure Olympic and Paralympic Games."
Those living in the Lexington building apartment complex in Bow received a leaflet over the weekend detailing the plans to use an old watch tower on top of their block of flats to host one of the weapons - sparking concerns.
They were told the weapons would be operated by 10 soldiers - guarded by police 24 hours a day - and only fired as a last resort.
One of the 700 residents of the block, journalist Brian Whelan, said on Sunday: "I've looked these (the missiles) up and I don't think they're the kind of thing you can fire over a highly-populated area like Tower Hamlets. Think of the debris."
No final decision has been made on any deployment of the ground-based air defence system.
General Sir Nick Parker, standing joint commander, said: "We have got to make sure that we have got a plan in place that can deal with the very unlikely, but serious threat that might exist to the Olympic Park."
He said any decision to actually fire the weapons would be taken "at the highest political level", adding that any military advice to the Government on the security provisions for the London 2012 Olympics would depend on the success of the trial.