Jacques Rogge and Lord Coe said the Games would survive the downturn
London's Olympics will help Britain out of the economic crisis, 2012 chairman Sebastian Coe has said.
Lord Coe said the Games, which start on 27 July 2012, protects 3,500 jobs on the Olympic site and creates contracts worth billions of pounds.
International Olympic Committee (IOC) member Kevan Gosper said organisers of the Games face the toughest economic circumstances outside of a war.
Lord Coe responded that the Games will have "extraordinary" economic impact.
Mr Gosper, a vice-president of the Sydney Olympics organising committee, said: "I think you and your team face the toughest time - short of war-time - to get the project to 2012."
But Lord Coe told a Royal Society of Arts, Manufactures and Commerce meeting that 10% of workers on the Olympic site in Stratford, east London, had been permanently unemployed.
"The work safeguards these jobs in an economic downturn," he said.
Lord Coe added: "No-one would have chosen this downturn but the Games could account for 6 to 7% of economic activity in this city over the next five years, not to mention the impact it could have on other parts of the country.
In good times or in bad this is a project that really has an extraordinary impact
Lord Coe, London 2012 chairman
"That's why we should be on the front foot - in good times or in bad this is a project that really has an extraordinary impact."
IOC president Jacques Rogge said he was confident the 2012 Olympics would survive the global economic downturn.
"The Games have survived difficult times before," he said. "They have survived and thrived because of what they mean to people all over the world.
"The Games remind us that the transient difficulties of life can be overcome through hard work and determination."
Mr Rogge said the 2012 Games would be less dependent on huge building projects than Beijing, using existing and temporary venues.
"It will only build facilities that will be valued and used by the local community long after the Games are over. London has made regeneration a priority," Mr Rogge said.
"Each Games are unique. It is not the amount of money spent that determines how good a Games is, it is also the unique and inspiring atmosphere created within the city. I'm sure London will do very well there."
Earlier this month Olympics minister Tessa Jowell told leisure industry bosses that "had we known what we know now" about the economy "we would almost certainly not" have bid for the Games.
But she later insisted that she had meant people may have seen a bid for the Games as a "distraction".