Demonstrators amassed on San Francisco's waterfront Wednesday as the Olympic torch made its only stop in North America. Meanwhile, law enforcement officers attempted to prevent a reprise of the chaotic demonstrations that have followed the flame along its journey to Beijing.

The torch's 85,000-mile, 20-nation global journey is the longest in Olympic history, and is meant to build excitement for the games. But it has also been a target for activists angered over China's human rights record, prompting officials to warn they might make a last-minute change to the relay route.

Thousands of people had gathered along the relay route, which hugs the San Francisco Bay. Pro-Tibet and pro-China groups were given side-by-side permits to demonstrate, and there already were signs of tension.

"A lot of Tibetan people are getting killed," said Kunga Yeshi, 18, who had traveled here from Salt Lake City. "The Chinese said they'd change if they got the Olympics, but they still won't change."

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Across the street, a bus carrying dozens of pro-China supporters arrived.

"The Olympic spirit unites all human beings, not only Chinese — also America and the whole world," said Hui Chen, 36, of San Jose. Chen said his was one of 50 buses chartered by an amateur sports association that wanted to celebrate China's first opportunity to host the Olympic Games.

As runners carry the torch on its six-mile route, they will compete not only with people protesting China's grip on Tibet and its support for the governments of Myanmar and Sudan, but also with more obscure activists. They include nudists calling for a return to the way the ancient Greek games were played.

One runner who planned to carry the torch dropped out earlier this week because of safety concerns, officials said.

Local officials say they support the diversity of viewpoints, but have ramped up security following chaotic protests during the torch's stops in London and Paris and a demonstration Monday in which activists hung banners from the Golden Gate bridge.

"We are trying to accomplish two goals here. One is to protect the right to free speech and the other is to ensure public safety, and here in San Francisco we are good at both of those things," said Nathan Ballard, a spokesman for Mayor Gavin Newsom.

Ambulances were to be stationed along the torch's route, extra sheriff's deputies and state law enforcement officers were put on patrol. Vans were deployed to haul away arrested protesters, and the FAA restricted flights over the city to media helicopters, medical emergency carriers and law enforcement aircraft. Law enforcement agencies erected metal barricades and readied running shoes, bicycles and motorcycles for officers preparing to shadow the runners.

The flame was whisked to a secret location shortly after its pre-dawn arrival in San Francisco on Tuesday. It began its worldwide trek from Ancient Olympia in Greece to Beijing on March 24, and was the focus of protests from the start.

San Francisco was chosen to host the relay in part because of its large Chinese-American population.

International Olympic Committee President Jacques Rogge said the body's executive board would discuss Friday whether to end the remaining international legs of the relay after San Francisco because of widespread protest. The torch is scheduled to travel to Buenos Aires, Argentina, and then to a dozen other countries before arriving in China on May 4. The Olympics begin Aug. 8.

"We recognize the right for people to protest and express their views, but it should be nonviolent. We are very sad for all the athletes and the people who expected so much from the run and have been spoiled of their joy," Rogge said.

Meanwhile Wednesday, the White House said anew that Bush would attend the Olympics, but left open the possibility that he would skip the opening ceremonies. Asked whether Bush would go to that portion of the games, White House press secretary Dana Perino demurred, citing the fluid nature of a foreign trip schedule this far out and the many factors that go into devising it.

"I would again reiterate that the president has been very clear that he believes that the right thing for him to do is to continue to press the Chinese on a range of issues, from human rights and democracy, political speech freedoms and religious tolerance, and to do that publicly and privately, before, during and after the Olympics," she said.