When Rio was awarded the Games three years ago, it was hailed as a rite of passage for Brazil, Latin America's biggest country and an economy that is now the world's sixth largest.
Along with the World Cup, to be held in Rio and 11 other Brazilian cities in 2014, the Olympics would show that Brazil was finally reaching long-elusive first-world goals.
But the exuberant celebrations which greeted the decision to award Rio the Games are giving way to trepidation in this seaside metropolis of 6.5 million people.
Construction delays, cost overruns and overburdened airports, roads and subway lines give locals a sense that Rio, the first South American city to be awarded the Olympics, has a long way to go if is to stage the event as seamlessly as London.
Part of the unease has to do with the sense that Rio, despite its long history as a global attraction, is still playing catch-up with the developed world.
Even after a recent economic boom in Brazil, soaring investment because of the sporting events and an ongoing rush to develop massive new offshore oil fields south of the city's beaches, Rio remains pockmarked by poor development.
"Brazil and Rio have four years to do all those things that have not been done in 400," said Alberto Murray Neto, a Sao Paulo attorney and past member of Brazil's Olympic committee.
The task is huge. Brazil's tourism ministry expects almost 400,000 foreign tourists for the Games, in addition to hundreds of thousands of Brazilians who themselves will add to the crush on airports, hotels, roads and other infrastructure.
Meanwhile, costs for Olympic projects are soaring, as the investment boom and Brazil's high taxes and labour costs, known locally as the 'Brazil Cost', inflate the price of everything from construction cranes to beachside coconuts.
The cost of the Games, critics fear, could far exceed initial estimates of 29 billion reais (£9.2 billion).
Rio Mayor Eduardo Paes, who lands in the city after travelling from London with the official Olympic flag on Monday, said in a recent briefing that an updated budget isn't possible yet.
Luis Fernandes, executive secretary of the Brazilian sports ministry, also sidestepped the issue, telling reporters in London on Monday: "We can only disclose the cost of the Olympics when everything is ready.
"In this budget, there are certain aspects we have to take note, when it comes to sporting venues that will be prepared and constructed.
"Our horizon is to base ourselves in the main, original programme we proposed."
So far, very little is ready. During their last visit in June, members of the International Olympic Committee said "the timelines for delivery are already very tight and the amount of work to be completed is considerable".
Most troubling, said the IOC, is that Rio has yet to begin building the Olympic Park and complex of buildings that will host most competitions and media facilities.
Paes and other city officials remain upbeat. Leonardo Gryner, chief of the Rio organizing committee, in London last week said all sports facilities would be ready by 2015 with ample leeway for testing.
Rio is no newcomer to big events.
The city's famed carnival celebrations attract more than 800,000 revellers each year. Big concerts and New Year's festivities on Copacabana beach have attracted over one million.
Rio hosted the 2007 Pan American Games, though critics recall that event was also marred by cost overruns and a lack of lasting improvements.
On Monday, O Globo, Rio's biggest daily newspaper, featured a photograph of a dirty and tattered flag over a Pan American memorial, calling it "a portrait of abandon".
The Maracana, Rio's main football stadium, was rebuilt for the occasion, only to be razed again to be reconstructed for the 2014 World Cup.
Rio officials tout ongoing efforts to spruce up the city after decades of disrepair.
Until Brazil's recent boom helped begin reviving its fortunes, Rio suffered from a lack of investment, soaring crime rates, and the encroachment of favelas, the city's well-known shantytowns, into its verdant hillsides.
And despite recent progress, development hurdles remain.
Rio's airports, like those elsewhere in Brazil, are notoriously crammed and have strained with air traffic growth.
Plans for a high-speed rail link between Rio and Sao Paulo, Brazil's biggest city, are so far behind schedule that officials concede it won't be running in time for the Games.
The line would be a major step forward for a country with few long-distance passenger rail links and where long bus and car journeys, often over rickety roads, are the only alternative to flying.
And Rio's streets, already constrained by its hills and a wraparound waterfront, are clogged daily by traffic.
"Moving around the city is nearly impossible under normal circumstances," said Christopher Gaffney, an urbanism professor at Fluminense Federal University. "I don't see how they expect to add users."
So scarce are the city's hotel rooms, even after an additional 10,000 are built for the Games, that officials plan to use cruise liners for extra accommodation.
Hotel operators, like other industries hurt by Brazil's poor school system, are scrambling to find skilled workers.
Consumers, meanwhile, worry about price gouging.
Brazil's government this year had to step in and force Rio hotels to cut prices ahead of a major United Nations environment summit. Before the June summit, hotels were charging as much as five times the normal room rates.
Social problems also complicate planning.
New roads and rail lines being built to reach Olympic venues, in the city's far-flung southern suburbs, will run right through some of Rio's poorest neighbourhoods.
Residents of Vila Autodromo, a favela of 500 families, are among tens of thousands who could be evicted by construction of Olympic projects.
While a security crackdown has reduced violence in many neighbourhoods, the improvements are mostly along the coastal corridor where most of the Olympic-related activity will take place, displacing the problems to formerly quiet corners.
Gryner, the Rio committee chief, said that Rio had learned "a lot" from the London Games. "We are taking that back to our teams," he said, "we are improving our planning processes."
But critics fear only so much can be carried over from a first-world city to one where basic public services are often lacking.
"We are comparing a developed country with an under-developed country, which still has a lot to do," said Murray Neto, the former Brazilian Olympic official.