Three members of the Paris-based group Reporters Without Borders were detained after their protest, which occurred as Liu Qi, president of the Beijing Olympics organizing committee and Beijing Communist Party Secretary, was giving a speech. Police confirmed they had detained three French nationals.
One protester held a black banner showing the Olympic rings as handcuffs. The group said three members, including the group's secretary general Robert Menard, managed to get into the ceremony without being stopped.
"If the Olympic flame is sacred, human rights are even more so," the group said in a statement. "We cannot let the Chinese government seize the Olympic flame, a symbol of peace, without denouncing the dramatic situation of human rights in the country."
The group has urged heads of state to boycott the games' opening ceremony.
International Olympic Committee President Jacques Rogge attended the ceremony at the 2,800-year-old birthplace of the ancient games in southern Greece. The flame for the Aug. 8-24 games was lit using the sun's rays.
Greek officials said politics have no place at the event ahead of expected protests by pro-Tibetan groups. More than 1,000 police were deployed around the site.
Meanwhile, China pledged strict security measures to ensure that the torch relay, which begins with the lighting ceremony, is not marred by protests.
China state TV cut away from the protest and showed a prerecorded scene, preventing Chinese viewers from seeing the incident. Chinese television commentators did not mention the demonstration.
China's Communist leadership has faced a public relations disaster since demonstrations against Chinese rule turned violent March 14 in the Tibetan capital of Lhasa, sparking waves of unrest in surrounding provinces. People who sympathize with the Tibetan cause have also staged rallies in other countries.
The death toll from the violence has varied and been impossible to confirm independently. China's reported death toll is 22, but Tibet's exiled government says 80 Tibetans were killed. Another 19 died in subsequent violence in Gansu province, it said.
Rogge told The Associated Press on Monday that he was engaged in "silent diplomacy" with China on Tibet and other human rights issues. But he also said there was no credible momentum for a boycott and that while he was concerned by the violence in Tibet, the IOC could do no more than call for a peaceful resolution because it is a sports organization.
Among the groups planning to protest was Students for a Free Tibet. The group's director Lhadon Tethong said officers detained one Tibetan campaigner and a Greek photographer with him in the village of Ancient Olympia, just outside the site. "One of our colleagues saw them being dragged by about 20 police through town," Tethong said.
The ceremony was held an hour early - starting at 5 a.m. EDT - to avoid rain forecast for later Monday. An actress dressed as a high priestess lit the flame using a convex mirror to focus the sun's rays on the Olympic torch.
From Olympia, the flame will embark on an 85,000-mile journey. The torch is to arrive March 31 in Beijing. It then will travel through 20 countries before returning to mainland China.
Chinese media reported that officials - who have blamed the unrest on the exiled Tibetan spiritual leader, the Dalai Lama - were prepared to prevent a disruption of the torch relay.
China's plans to take the torch through Tibet and to the top of Mount Everest have upset Tibetan activist groups, which accuse Beijing of using the event to convey a false message of harmony in the troubled Himalayan region. Chinese Communist troops occupied Tibet in 1951 and Beijing continues to rule the region with a heavy hand.
"The more determined the Dalai clique is to ruin the torch relay and the Olympic Games, the more hard and good work we need to do on the preparation and the implementation of all aspects," Yin Xunping, a Communist Party official, was quoted as saying by the Tibet Daily newspaper.
Yin is party secretary of the Tibet Mountain Climbing Team, which is participating in the Mount Everest segment of the torch relay. He spoke at a meeting organized last week by Tibet's sports bureau, whose head, Dejizhuoga, urged "intense precautions and heightened security."
The report, cited Monday by the official Xinhua News Agency, did not give any details of what measures would be taken. A receptionist at the Tibet sports bureau said no officials were available for comment Monday.
Mount Everest straddles the border between Nepal and Tibet. China has already begun denying mountaineers permission to climb the Tibetan side of the mountain - a move that reflects government concerns that activists may try to disrupt its torch plans.
The first torchbearer in the relay will be Greece's Alexandros Nikolaidis, who won a silver medal in taekwondo at the 2004 Athens Games. He will hand the flame to Luo Xuejuan, who won China's only swimming gold medal in Athens.