A massive security operation is under way in San Francisco, as the city prepares to host the only US leg of the international Olympic torch relay.
Barricades have been set up and streets sealed off close to the planned relay route. Police leave has been cancelled.
People protesting against China's policies are gathering - as are pro-Beijing members of the city's large Chinese population.
There were chaotic scenes as the torch passed through London and Paris.
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But correspondents say more trouble could force officials from the International Olympic Committee to consider calling off the rest of the torch's 136,788 km (85,000-mile) journey.
No-fly zone
"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 San Francisco Mayor Gavin Newsom.
Hundreds of anti-Beijing protesters rallied in the city on Tuesday, after activists hung banners from the city's famed Golden Gate Bridge on Monday.
Police are out in force along the planned seafront route of the torch, and have declared the airspace above a no-fly zone.
Map of planned world torch relay route
But they have also cautioned that the route of the torch may be subject to last-minute change.
Tibet is not the only issue prompting the protests - China's policies towards Sudan and members of the banned Falun Gong spiritual movement are also being opposed.
At Tuesday's rallies, South African Archbishop and Nobel Peace Prize laureate Desmond Tutu urged world leaders not to go to the Games. Hollywood actor and long-time Tibet activist Richard Gere attacked China's plans to parade the torch through Tibet.
But in San Francisco's Chinatown, community representatives held a news conference to call for a peaceful relay and voice pride over China's hosting of the Games.
The flame was lit in Greece on 24 March and is being relayed through 20 countries before being carried into the opening ceremony in Beijing on 8 August.
Protests have already caused serious disruption to legs in London and Paris. In Paris, the torch had to be extinguished three times, while in London there were 37 arrests.
Most demonstrators are protesting over a security crackdown in Tibet after anti-Chinese unrest.
Tibetan exile groups say Chinese security forces killed dozens of protesters. Beijing says about 19 people were killed in rioting.
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(BBC)
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