Olympic banner to Rio as the next Games host city.
The statue of Christ the Redeemer is seen with the Sugar Loaf mountain at the back in Rio de Janeiro, Brazil.
The Brazilian Olympic Organizing Committee and city authorities insist that all the planned infrastructure projects have already begun, although the Olympic village that will welcome some 14,000 athletes and the Olympic park that will host nine events have yet to get off the ground.
There is still no executive master plan and no clarity about how much staging the world's biggest sporting event will cost.
The major challenges, according to experts, are transportation, infrastructure and accommodation.
This city of 6.5 million people expects to welcome more than a million tourists and more than 10,500 athletes over 15 days but will have only 34,000 hotel rooms for 2016.
And it is notorious for its massive traffic jams.
Some of the infrastructure projects for the 2016 Olympics will be ready for the 2013 FIFA Confederations Cup and the 2014 World Cup Brazil is due to stage, but much work still remains undone.
"Now we are really going to take the measure of the complexity of organizing an event such as the Olympics," marketing analyst Erich Beting told AFP.
"Over the past three years, we have not really focused on beginning the construction of the Olympic city... and in creating the necessary infrastructure."
Leonardo Gryner, director general of the Rio 2016 organizing committee has admitted that his main worries are transport and hotels.
There are planes to extend a metro line until the Barra da Tijuca district in the western sector and four express lanes for buses and cars are also under construction.
Many of the urban transit projects "are generally running behind schedule and the government is aware that some of the projects will not be ready in time for the 2014 World Cup, and some not even for the 2016 Olympics," said Carlos Campos, an economist and analyst at the Applied Economics Research Institute.
Gryner said about 47% of the sports installations needed for the Games already exist, including some built for the 2007 Pan American Games.
About 28% will be new and 25% such as the grandstands on Copacabana beach for the beach volleyball competition will be temporary.
Rio Mayor Eduardo Paes, who is seeking re-election in October, said the deadline for infrastructure projects was being met.
But he added that the total cost was not known.
"It can vary. We will only begin to disclose the costs of the Rio Olympic Games once we have an executive project ready," Paes said.
When the city put forward its candidacy, it forecast investment of 28 billion reals ($14.4 billion) -- 23.2 billion real in public and private resources and 5.6 billion real from the Organizing Committee.
Once considered one of the most violent South American cities, Rio on Sunday is considerably more secure since authorities launched a drive in 2008 to wrest control of dozens of slums from drug traffickers.
Nearly 150 of the total 750 favelas have been pacified by the military or police.
However, "Rio is a violent city compared with those in Europe, the United States or regional capitals," Ignacio Cano, a researcher at the Violence Analysis Center of Rio State University.
President Dilma Rousseff expressed hope that the Games will be a boon for the country.
"We want the best possible legacy from the Games for Brazil, both from a sports point of view and in terms of a better infrastructure and quality of life for the population," she added at the London Games.