The Olympic torch has been lit at a ceremony in Greece that was briefly disrupted by pro-Tibet activists.
Protesters from media rights group Reporters Without Borders broke through the cordon of 1,000 police officers in Olympia as China's envoy spoke.
Activists had vowed to protest over the violence in and around Tibet.
The torch will now be carried in an around-the-world relay through 20 countries, before arriving in Beijing for the start of the Games on 8 August.
As Liu Qi, head of the Beijing Olympic organising committee, spoke ahead of the torch lighting, three men broke into the ceremony venue.
One ran up behind him attempting to display a black flag depicting the Olympic rings made from handcuffs.
The men were from the France-based media rights watchdog Reporters Sans Frontieres (Reporters Without Borders, or RSF), which has called for a boycott of the opening ceremony of the games.
They were quickly bundled away by police and Mr Liu continued his speech uninterrupted.
The live television coverage, beaming the scene around the world, quickly cut away from Mr Liu and the protesters until they had been removed.
"We cannot let the Chinese government seize the Olympic flame, a symbol of peace, without denouncing the dramatic human rights situation in the country," RSF said in a statement.
Later, as the torch began its journey, pro-Tibet activists unfurled banners and shouted slogans before Greek security wrestled them away.
Actors dressed in ancient Greek costume then lit the torch in the traditional manner by using a parabolic mirror to focus the sun's rays.
Despite cloudy weather, the torch was lit using the sun's rays
There were fears that stormy weather would prevent the torch being lit in the customary way.
The ceremony, beside the Temple of Hera, was moved forward by an hour and the sun shone through a break in the clouds.
The head of the International Olympic Committee (IOC), Jacques Rogge, has defended the decision to hold the Games in China saying: "The major political leaders don't want a boycott".
In his speech during the ceremony, Mr Rogge said the Olympic torch relay and the Games should take place in a peaceful environment.
"The torch is the link between all athletes and citizens of this world; between all of us who believe in Olympism and the virtue of sport. It has the force to unite humanity and to stand for harmony."
He told the Associated Press news agency on Monday that he was engaged on a daily basis in "silent diplomacy" with Beijing on Tibet and other human rights issues.
Tibet activists are angered that the torch's 136,000-km (85,000-mile) route will take it through the Himalayan region and Mount Everest, which straddles the border between Nepal and Tibet.
China sent troops to Tibet in 1950 and since then there have been periods of unrest and sporadic uprisings as resentment of Beijing's rule has persisted.
The latest round of anti-China protests began in Tibet's main city, Lhasa, on 10 March - the 49th anniversary of a failed uprising - and gradually escalated.
Lhasa saw at least two days of violence and there have also been protests in provinces which border Tibet.
China says 19 people were killed by rioters and accuses Tibetan spiritual leader the Dalai Lama of inciting the violence.
The Tibetan government in exile says at least 130 people have died in a crackdown by Chinese troops and deny any role in the protests.
OLYMPIC TORCH ROUTE
Torch lit in Olympia on 24 March and taken on five-day relay around Greece to Athens
After handover ceremony, it is taken to Beijing on 31 March to begin a journey of 136,800 km (85,000 miles) around the world
Torch arrives in Macao on 3 May. After three-month relay all around China, it arrives in Beijing for opening ceremony on 8 August