RAF Typhoon jets arrived at an air base in London today 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 from today 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.
"There is no specific threat and all we are doing is having in place what we would describe as prudent and appropriate measures in place, in order that we could react if required in a timely and appropriate fashion."
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.
Other aircraft involved in the exercise include Royal Navy Sea King helicopters temporarily based at RAF Northolt, RAF Puma helicopters based at a Territorial Army centre in Ilford, east London, and Army and Royal Navy Lynx helicopters on HMS Ocean in the Thames.
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.
"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."
Squadron Leader Gordon Lovett, who was among the pilots arriving in west London today, said everyone who will operate during the Games will fly in the exercise.
"There's the emotional part of it, the stress level, that you cannot do in a simulator which is why we've brought the guys down here to make sure they are happy working out of here in a calm environment, not worrying about an operational issue.
"So that when they go and do an operational issue they aren't worried about the airspace, the airliners, etc."
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 and not forced on ordinary people in east and south London who have no say in the matter."
Last month a sonic boom caused by two Typhoon aircraft responding to an emergency signal was reportedly heard as far afield as Bath, Coventry and Oxford.
Group Captain Tim O'Brien, station commander of RAF Northolt, said he hoped people living nearby would understand the need for the Typhoon jets, which can travel at up to 1,370 miles per hour.
Discussions have taken place with residents, and signs have been placed by roads to warn motorists of loud noises.
"There's very much a sense of cohesion with the local populace that they are in this with us, and they are going to reap the benefits of the mutual pride that we will gain from it as well," Group Capt O'Brien said.