Also clouding the picture today was a thick blanket of smog that has hovered over the city for weeks — not the blue skies hoped for by the organizers of the Beijing Games.

Officials including International Olympic Committee President Jacques Rogge will mark the start of the one-year countdown with a lavish ceremony tomorrow in Tiananmen Square.

But today, Chinese authorities detained six activists descending part of the Great Wall with a 450-square-foot banner reading, "One World, One Dream, Free Tibet 2008."

The London-based Free Tibet Campaign and Students for a Free Tibet said China is using the Olympics to strengthen its claims on Tibet. China says it has ruled the remote region for centuries but many Tibetans say their homeland was essentially an independent state for most of that time. Chinese communist troops occupied Tibet in 1951, and Beijing continues to rule the region with a heavy hand.

In an open letter to Rogge and Chinese President Hu Jintao and Premier Wen Jiabao, a group of 40 well-known dissidents said China's Olympic preparations had obscured widespread civil-rights abuses.

"We find no consolation or comfort in the rise of grandiose sports facilities, or a temporarily beautified Beijing city, or the prospect of Chinese athletes winning medals," the letter said.

Chinese officials promised foreign reporters free movement seven months ago. However, the temporary freedoms do not extend to local Chinese journalists.

The Committee to Protect Journalists urged Beijing to free 29 imprisoned journalists and loosen the restrictions on local reporters. It also called on the IOC to pressure China to increase press freedoms.

"There's a major responsibility for the IOC ... to speak out and to encourage the government of China to live up to its commitments," said Wall Street Journal editor-at-large and CPJ board chairman Paul Steiger.

The IOC said the games, which begin Aug. 8, 2008, could be a "catalyst for constructive dialogue."

"We believe that the Olympic Games will act as a vehicle for positive change in many ways, some of which may take time to realize," the IOC said. "From what we have learnt in meeting with human rights groups, they typically agree with our viewpoint that the Games being held in Beijing is a positive thing."

Rogge has called the games "a force for good" but has cautioned they cannot change China's political and social institutions.

Dismal air quality, meanwhile, could threaten the performance and health of thousands of elite athletes, and some are already saying they will stay away until the last minute because of pollution concerns.

Australian IOC member John Coates said after meeting today with other Olympic delegations and Beijing organizers that he'd be telling his athletes to stay away from Beijing until four or five days before their events.

He called Beijing's choking pollution "a prevailing worry for most of us."