The countdown to the Athens Games began in earnest on Thursday when the Olympic flame was ignited in Olympia.
Greek actress Thalia Prokopiou, dressed as the high priestess, lit the torch at the Temple of Hera in a recreation of the ancient Olympic ceremony.
For the first time in history, the flame will visit all five continents represented by the Olympic rings on a 78-day world tour.
The torch relay will begin with a seven-day trip through southern Greece.
Greek former javelin medallist Costas Gatzioudis was the first torch-bearer, handing over to Russian swimmer Alexander Popov.
Popov won two sprint gold medals at the 1992 Olympics, two in 1996 and a silver in 2000.
Other prominent runners on the first few days include Prince Albert of Monaco, who took part in former Winter Olympics, and legendary pole vaulter Sergei Bubka.
The flame will burn outside the marble Panathenian stadium, site of the first modern games, until 4 June.
It will then embark upon its world tour, with the Australian city of Sydney the first stop.
The flame will make its first trips to South America and Africa, and visit the United States from 16-19 June.
Its route will include all of the nine cities bidding for the 2012 Olympics - London, Paris, Madrid, New York, Rio de Janeiro, Havana, Moscow, Istanbul and Leipzig.
It is due in Paris on 25 June and in London the following day, where it will make an appearance during the Wimbledon championships.
Athens is facing a race against time to deliver the Games following delays in the contruction of key Olympic sites and transport networks.
But the lighting ceremony itself went off without a hitch, with the torch bursting into flame as it was held inside a parabolic mirror and ignited by the sun's rays.
Prokopiou said a prayer to the ancient Greek god Apollo for the sun to shine.
The ceremony was preceded by an address from the International Olympic Committee president Jacques Rogge and organising committee president Gianna Angelopoulos-Daskalaki.
"Today the Olympic flame will be reborn yet again to enfold the whole world in its light," said Angelopoulos-Daskalaki.
"This is the day that all of us have been waiting for so eagerly."
Rogge added: "The Olympic Games are returning to their country of origin, for it was here in Olympia that everything began in 776 BC.
"And today everything is going to begin again.
"More than ever before, Athens and Greece will be the focus of the attention for the whole world.
"What better image can the organising committee offer than to organise the first ever global Olympic torch relay.
"Whether we live in a rich or disadvantaged country, the flame will unite us all."