At an Athens ceremony featuring ancient Greek goddess costumes, Brazilian organizers took the Olympic flame for the start of a journey that will see it carried by 12,000 people around Latin America's biggest country.
"Rio is ready to make history," Carlos Nuzman, president of the Rio 2016 Organizing Committee, said at the torch handover.
New Zealand athletes marked the 100 day countdown with a traditional Haka dance on an Auckland beach at sunrise. Buildings around the world lit up in special colors -- Brazilian yellow and green at Tokyo's Municipal Government Headquarters and US red-white-blue for the Empire State Building in New York.
With Rio's hills, beaches and the famed statue of Christ the Redeemer providing one of the most telegenic backdrops in the world, the Games are expected to be spectacular.
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But despite the insistence of the Brazilian government and International Olympic Committee that everything will be ready, storm clouds are gathering.
President Dilma Rousseff appears likely to be suspended from office through impeachment in the next few weeks. Even her vice president, Michel Temer, who would normally take over -- and whom she accuses of mounting a coup -- could face action.
Crime remains out of control. Brazil's human rights record came under fire today with a demand by Amnesty International for action over rising police killings, particularly in Rio's favela shanty towns.
The rights group said 11 people were killed in police shootings in Rio in just the past month and at least 307 people were killed by police in the city last year -- amounting to 20 per cent of all homicides.