What has raised concerns in London is that Britain has been listed by its own security and intelligence agencies as being at high risk of a catastrophic attack by Islamist militants, partly because of its alliance with the United States in the wars in Afghanistan and Iraq, but partly, too, because of the large number of terrorist cells known to be active in the communities where Britain’s one and a half million Muslims live.

R.A.F. officers say that with years to plan since the Games were awarded to London in 2005, they have had time to fashion a shield against not only hijacked airliners, but also against an array of other airborne threats. These, they say, include light aircraft, hot-air balloons, fixed-wing gliders, hang gliders, microlight planes, airships, unmanned drones and even remote-controlled model aircraft capable of carrying small bombs.

The main defense against lighter, lower-altitude and slower threats consists of sniper-carrying helicopters based at an army base in Ilford, east of the Olympic Park, and aboard the Ocean, a Royal Navy helicopter carrier moored in the Thames a couple of miles south of the park. Military commanders believe that with the Typhoons, the Puma and Lynx helicopters and the Starstreak missile batteries, they can deal with any airborne threat, at any altitude, in minutes.

The Royal Air Force’s state of readiness was demonstrated last week when a Typhoon was scrambled from Northolt to intercept a Boeing 757 airliner flying at 30,000 feet over the coast of Brittany, in northern France, after the aircraft failed to respond to British air traffic controllers for a few minutes. The aircraft, carrying more than 240 passengers and crew members on its way from Tunisia to Glasgow, Scotland, finally identified itself as it flew over the English Channel. By that time, the Typhoon, climbing above 15,000 feet less than a minute from takeoff, was well on its way to challenge the airliner.

Normally, fighters on standby are based at airfields on Britain’s east coast, flying “quick reaction alert” missions that intercept the four-engine Russian Bear bombers and airborne spy planes that continue to run occasional training missions that approach British-controlled airspace, 20 years after the cold war ended. Olympics flight security is far more difficult, Squadron Leader Lovett said, not least because the restricted flight zone around the Olympic Park, reaching out about 35 miles, includes three major civilian airfields — principally Heathrow Airport, with about 1,200 flights a day — making London’s skies among the busiest in the world.

The main challenge, the pilots say, is to anticipate and adapt to what would-be terrorists might do and deny them the element of surprise that aided the 9/11 attackers.

“It’d be a mistake to think we know everything the potential terrorist might come up with,” said Wing Cmdr. Shane Anderson, 42, the Australian-born leader of the R.A.F.’s 33 Squadron, flying Puma helicopters out of the Ilford base. “What we have to do is to be adaptive, and achieve intellectual overmatch.” In plain terms, the aircrews and mission controllers have to outthink the enemy.

“We have to plan for the extreme,” he said.