RAF Typhoon jets have arrived at an air base in London in preparation for the Olympic Games as military chiefs said they were ready to react to a "9/11-type attack".
The four high-speed jets landed just before midday at RAF Northolt in west London ahead of a major military exercise to test security for London 2012.
It is the first time fighter aircraft have been stationed at the base since the Second World War.
The Typhoons will take part in Exercise Olympic Guardian, a nine-day training operation over the skies of the capital and the home counties that runs until May 10.
Air Vice-Marshal Stuart Atha, air component commander for Olympics air security, said: "What we will have is a plan that has many levels to it which will allow us to deal at one end - which is that 9/11-type attack - perhaps down to the lower and the slower type of threat that we may face."
Air Vice-Marshal Atha said he hoped the exercise would have "an effect on the mind" of any potential attackers. "I would hope when they see how we are preparing they might be deterred from making any threats to the Games," he added.
The RAF warned that people in south-east England will notice an increase in air activity at certain times, in particular this weekend.
Defence Secretary Phillip Hammond said: "The fact that our state-of-the-art Typhoons will be stationed at RAF Northolt underlines 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 said such a heavy military presence in London was "unacceptable" and warned it would create a "climate of fear".
The group's Lindsey German 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. If the Olympic Park needs security, this should be within the confines of the park."