BEIJING — As China prepares to dazzle the world at the Beijing Olympics, an uninvited guest is showing up early and threatening to mar the spectacle: free speech.

A political row erupted in Europe when some Olympic committees and officials tried to muzzle athletes from speaking out about human rights and other sensitive issues at the games.

Hollywood director Steven Spielberg quit as an artistic adviser to Beijing this week because he thought China wasn't doing enough to pressure Sudan to end the conflict in Darfur. Nobel laureates and former Olympic gold medalists added their voice on China's role in the humanitarian crisis by issuing an open letter.

The developments illustrate just how rocky the road to the Beijing Olympics is likely to be. With the event six months away, the world is taking a harder look at China's persisting ills — while the communist government is hoping the Olympics will redefine the country as a welcome global power.

The message China hopes to convey: "Look at us, we're a big friendly panda, not a big scary dragon," said David Wolf, a Beijing-based media consultant.

The current discord has challenged Beijing's meticulous plans for the games. The government has spent tens of billions of dollars making over the Chinese capital with architecturally stunning venues and infrastructure. The Beijing Olympics theme — "One World, One Dream" — carries a message of harmony.

But the combination of an authoritarian government with big Olympic ambitions on one side and a plethora of vocal critics on the other seems likely to make this summer's games among the most politicized in decades.

Politics has intruded before: The United States led a boycott of the 1980 Moscow games to protest the Soviet invasion of Afghanistan. That prompted a Soviet-led boycott of the Los Angeles Olympics in 1984.

"No matter Beijing likes it or not, politics has been important part of the games from the very beginning," Xu Guoqi, a China-born historian at Kalamazoo College in Michigan, said in an e-mail. "The problem is that Beijing hates to be cornered by the critics of China who use the games to score politically or diplomatically."

And China's critics are numerous.

Britain's Prince Charles says he won't go to show support for the Dalai Lama, exiled from Chinese-controlled Tibet where he once reigned. World-class athletes have talked of wearing masks to block out Beijing's choking pollution. Human rights and media watchdog groups point with dismay to the practice of jailing dissidents and journalists on questionable charges.

In recent days the British and Belgian Olympic committees drew fire from rights groups after trying to ban athletes from making politically sensitive remarks or gestures during the Olympics. The British Olympic Association backtracked, saying it only wanted athletes to respect the Olympic Charter, which prohibits demonstrations and "political, religious or racial propaganda" at Olympic sites.

On Darfur, where Sudanese government forces and militias have fought rebels in a conflict that has left more than 200,000 people dead, campaigners want China to use its heft as a big buyer of Sudanese oil to push the government to end the violence. Spielberg said Tuesday his conscience would not allow him to act as artistic adviser to the opening and closing ceremonies.

Always prickly when it comes to foreign censure, Beijing has lashed out, accusing critics of injecting politics into sports. In a typical comment, Foreign Ministry spokesman Liu Jianchao said Thursday that some detractors of China's policy on Darfur "may have ulterior motives, and this we cannot accept."

Preparations were progressing smoothly and "we believe excellent ceremonies will be presented to the world," the Beijing Olympic organizing committee, BOCOG, said in a brief statement. It noted that linking sports and politics was "not in line with the Olympic Spirit."

With half a million or more foreigners expected in Beijing this summer, organizers will have to be alert for the unexpected. Groups critical of China's rule in Tibet and religious restrictions have said they want to join the throngs.

Beijing must be ready for the possibility of, say, "a Tibetan monk setting himself on fire in front of the Beijing Hotel," said David Zweig, a China expert at Hong Kong University of Science and Technology.

"They have to think about it very seriously. They are not very adept at handling street protests by foreigners. They've never done it," he said.

In a sign of its nervousness, Beijing has decided to restrict media access to Tiananmen Square — the heart of a pro-democracy movement crushed by the military in 1989 — especially for television crews. International Olympic Committee officials say privately it's because of concerns about demonstrations. BOCOG officials worry that the Olympic torch relay is vulnerable, particularly outside China.

As the games approach, even minor disputes are likely to be magnified by the glare of attention, potentially knocking Beijing further off-message.

"Can you ensure that the sideshow doesn't move to the center stage? That's always a challenge for the Olympics and that will be the challenge for the Chinese this year," said Wolf, the media consultant.

One advantage for the Chinese leadership is its control over the media in China. That allows it to keep bad news from ordinary Chinese, who are largely supportive of the Olympics.

News of Spielberg's exit went mostly unreported for two days, until after the Foreign Ministry issued a response defending China's actions in Darfur. Beijing residents, who are eager to show off their city to foreigners, shrugged it off.

"Spielberg's action will have only very limited impact on the games," said Zhang, a 64-year-old Beijing resident who gave only his surname as is common in China. "Maybe he's an important figure in U.S., but here his words carry little weight."