LONDON — So, after twice organizing the Summer Olympics on short notice, London finally has the chance to show the world what it can do with a proper timetable.


In 1908, London stepped in for Rome after Mount Vesuvius erupted two years earlier. In 1948, London put on the Games after World War II in a city where food, clothing and gasoline were still being rationed and where the Olympic family, as it is now called, was housed in everything from Royal Air Force camps to college dormitories to tents.


Those Games were called “The Austerity Olympics,” but despite Britain’s recent economic struggles, the Games of the 30th Olympiad that will begin in London on Friday will be far from austere and very much in keeping with the big-budget, big-spectacle approach that has transformed the Summer Games into the world’s largest quadrennial showpiece.


“I used to joke that I would hate to be the city to have to follow Beijing, because of the resources and everything the Chinese put behind it,” said Michael Payne, a former International Olympic Committee executive, referring to the 2008 Games in China. “But I think in the end London will more than hold its own against any previous Games. The only black cloud for me is the security agenda and whether there is some crazy, as they say, lone wolf out there.”


Security costs, by some recent estimates, will exceed £550 million, or $860 million, for the London Games and have become an increasingly large part of the estimated total expenditure of £9.3 billion. Closed circuit televisions are as much a part of the British landscape now as roundabouts, and the country is well-accustomed to staging major sporting and cultural events, including Queen Elizabeth II’s Diamond Jubilee celebration last month.


But security has remained a concern in the weeks leading up to these Games with the inquiry into the private security firm G4S and its failure to fulfill its contractual commitment to providing sufficient staffing.


Londoners surely needed no reminder about the dark side of staging the Olympics. On July 6, 2005, they celebrated in pubs and in private, and at a party in Trafalgar Square, when London defeated Paris by a narrow margin to be named the host city for 2012.


The next day, terrorism shattered the festive mood as a coordinated bombing attack in the London Underground and on a double-decker bus left 52 people dead and hundreds wounded.


Though there has been no definitive linkage between the timing of that attack and the Olympic bid, the two remain linked in many Londoners’ minds, and there have been other security concerns in London since then, including the rioting and looting last summer.


Sebastian Coe, the former Olympic champion and chairman of the London Organizing Committee, categorized that at the time as “quite an aberration” and “some fairly unstructured late-night shopping.” But it was yet another reminder of how quickly mood and impressions can shift.


Mitt Romney, the presumptive Republican presidential nominee from the United States, used the term “disconcerting” in comments to NBC News to describe reports of the G4S staffing issues and of a potential strike by British immigration and customs officials.


“That obviously is not something which is encouraging,” said Romney, who was the chief of the organizing committee at the 2002 Winter Olympics in Salt Lake City, Utah.


In the same NBC interview, Romney also questioned the level of public support for the Olympics in Britain. “Do they come together and celebrate the Olympic movement?” asked Romney, who is in London and was expected to attend the Opening Ceremony on Friday. “That’s something which we only find out once the Games actually begin.”


On Thursday, David Cameron, the British prime minister, visited the Olympic Park in Stratford in East London and responded to Romney’s comments.


“This is a time of some economic difficulty for the U.K., but look at what we are capable of achieving as a nation even at a difficult economic time,” Cameron said.


During a speech in front of the new Olympic stadium, Cameron also said, “You’re going to see beyond doubt that Britain can deliver.”


He added: “We’ve delivered this incredible Olympic Park on time, on budget and in real style.”