LONDON - London, an ancient city steeped in theater and pageantry, stages its third summer Olympics against a sombre backdrop tempering the excitement and anticipation before the world's greatest sports festival.

Euphoria in Singapore, where London secured the Games in 2005, was succeeded within 24 hours by horror in the British capital when 52 commuters were killed by four suicide bombers.

Consequently the Games of the XXX Olympiad, opening at the Olympic stadium in east London on Friday, will feature Britain's largest peacetime security operation which has further inflated the budget in troubled economic times.

"The security threat comes from a number of diverse actors ranging from al-Qaida affiliated terrorists, lone wolves acting independently, Irish Republican Army offshoots as well as anarchists," said an assessment from the Asia-Pacific Foundation, a London-based think tank.

"Another potential danger is street violence similar to the wild, apparently spontaneous and totally unpredictable riots, that hit parts of London for five days in August 2011, causing hundreds of millions of pounds in property damage."

London's creaking transport system, criticized as "often obsolete" by International Olympic Committee inspectors after the city was short-listed for the Games, is another problem.

So, too, is the unpredictable British climate during a summer of unrelieved gloom and drenching rain, although signs that the weather gods may have finally relented came at the weekend with clear skies, rising temperatures and optimistic forecasts.

On the plus side, the Asia-Pacific Foundation report pointed out that Britain has a "very successful track record" in hosting major sports events, a point stressed by London 2012 Olympics organizing committee chairman Seb Coe.

"You do not want people coming to London feeling they have come to a siege town," Coe told Reuters. "London stages these global events all the time and often at the same time. We have a good track record but I am not being remotely cavalier or particularly sanguine about the nature of what we have to do."

London held the 1908 Olympics at the height of the golden Edwardian summer when Britain ruled much of the world. The 1948 Games were hosted by a city ravaged by wartime bombing in an emotionally and physically exhausted country burdened by a record debt.

In a link with the past, the Olympic Park is located in an area of London which suffered some of the worst bombing in World War Two, as part of a program of economic regeneration.