* Missiles placed on top of buildings for Olympics exercise
* Some residents demand rethink, worried about safety
* Britain prepares for biggest peacetime security operation
British military chiefs defended plans on Monday to put missile batteries on top of apartment blocks to help protect London from a 9/11-style attack during this summer's Olympic Games, after appalled residents said it could make them a target.
With 88 days to go before the Games start, soldiers will start testing missile defences this week at six sites around the Olympic park as part of a training exercise in the run-up to Britain's biggest peacetime security operation.
People living in one of the buildings earmarked for one of the missile batteries said they feared terrorists might attack their block and they were scared of the effects of shooting down a plane over a built-up area.
However, military planners said they had to beef up security to cope with anything on the scale of the Sept. 11, 2001 strikes or a smaller, "low and slow" strike by a single light aircraft.
"We are practising for the worst case scenario, not the most likely," General Sir Nick Parker, in charge of the military's Olympics role, told a news conference. "I do understand that this is unusual and people will be concerned. But for the greater good, it is prudent for us to provide this sort of air security plan."
As one of the biggest supporters of the U.S. invasions of Iraq and Afghanistan, Britain has long been regarded as a prime target for terrorists. Suicide bombers killed 52 people in co-ordinated attacks in London on the day after the city was awarded the Games in July 2005.
Britain currently rates the threat of a terrorist attack as "substantial", the third highest level, which means a strike is seen as a strong possibility.
Residents living near one of the planned missile batteries complained they had not been consulted and questioned the wisdom of siting them so close to homes.
"I can't imagine the circumstances that would require you to fire missiles over a highly populated area," Brian Whelan, a 28-year-old journalist, told Reuters.
"SAFE AND SECURE"
The Ministry of Defence said a final decision on whether to install missiles during the Games would be taken by Prime Minister David Cameron's government after the end of the Olympic security exercise, which runs from Wednesday to May 10.
Defence Secretary Philip Hammond said he hoped the public would be reassured by the extra security.
"The majority of this exercise will be played out in full view of the public and I hope 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," Hammond said in a statement.
During the exercise, fighter jets will move to an airbase in northwest London and a helicopter-carrying ship will be berthed at Greenwich, close to some of the Olympic venues and the Canary Wharf financial district.
Around 13,500 soldiers will help the police during the Games, more than the 9,500 UK troops serving in Afghanistan.
London's police force said it was preparing for terrorist attacks, organised protests and lone demonstrations, similar to the one which disrupted Britain's annual university boat race.
Some 12,500 police will be on Olympics duty each day across the country. Most will not carry a gun, in line with the long tradition of the British police.
Police leave and training courses have been cut during the Games, which run from July 27 to Aug. 12, to ensure that there are enough officers to cope with non-Olympic threats. Last summer, rioting erupted in several English cities after police shot dead a man in London.
"This is without doubt the biggest peacetime policing operation we have ever had to deal with," said Metropolitan Police Assistant Commissioner Chris Allison, in charge of Olympics security. "It will require a massive effort."
