Countdown begins

A British flag flutters next to a cauldron with the Olympic Flame atop the Athens Acropolis May 17, 2012.[Photo/Agencies]

The relay will travel 12,800 km around Britain and Ireland, taking in 1,018 villages and the 1,085-meter summit of Snowdon, before culminating with the lighting of the Olympic cauldron in the new stadium in east London.

"Millions of people across the UK are getting ready to do the best work of their lives to welcome you all," said Coe, adding a similarly warm reception would be felt when the flame makes a brief trip to Ireland.

"I know the Dubliners will show us how to celebrate the Flame and its message of peace, goodwill and friendship."

Johnson, re-elected mayor of England's capital earlier this month, said the reality of hosting the biggest show on earth had finally hit home.

"The countdown begins now. This isn't going to happen again in our lifetimes," he told the BBC.

Greek president Karolos Papoulias, whose debt-stricken country risks bankruptcy and an exit from the European single currency, also attended the ceremony after naming a caretaker Prime Minister in an emergency government on Wednesday to lead Greece to new elections next month.

Greek rowing world champion Christina Giazitzidou carried the flame into the stadium, built in 330BC and reconstructed for 1896, with a branch from what is claimed to be the oldest olive tree in the world in her other hand.

As she entered, as if by celestial command, the rain eased and the sun tried to break through the cloud.

Greek veteran weightlifter Piros Dimas (R) and Chinese gymnast Li Ning hold an Olympic Torch and a branch of olive tree after they light lit a cauldron with the Olympic Flame inside the marble Panathenaic stadium in Athens May 17, 2012. [Photo/Agencies]

The final two torchbearers were Greek weightlifter Pyrros Dimas and Chinese gymnast Li Ning, who lit the cauldron at the 2008 Beijing Games, running together with the torch.

Cries of 'Hellas, Hellas' went up from the crowd as the cauldron in the center of the stadium was lit and five white doves were released by school children.

The relay will take in landmarks around Britain with the flame travelling by canal boat, cable car, tram, steam train, hot air balloon and even motorcycle sidecar on the Isle of Man TT course.

More than 95 percent of the British population will be within an hour of the route.

"We are reminded of our responsibility - like that of our predecessors in 1908 and especially in 1948 - to stage Games that use the power of sport to unite the world in a celebration of achievement and inspiration in challenging times," said Coe.

"A Games that will inspire the next generation to choose sport."