Barack Obama has urged US President George W Bush to consider a boycott of the opening of the Beijing Olympics unless China's rights record improves.
The Democratic presidential hopeful made his call a day after a similar appeal by his rival, Hillary Clinton.
The US stage of the Olympic torch relay passed off amid confusion and tight security in San Francisco on Wednesday.
The route was changed at the last minute and the closing ceremony took place on a motorway fly-over.
Throughout the route, the torch-bearers were immersed in a cocoon of security, surrounded by dozens of police officers and track-suited Chinese guards.
Map of planned world torch relay route
The Olympic 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.
Demonstrators sought to disrupt the torch relay in Athens, Istanbul, Paris and London, while it passed successfully through Almaty, in Kazakhstan, and St Petersburg, in Russia.
It is due to arrive in the Argentine capital, Buenos Aires, later this week.
However, the International Olympic Committee is meeting in Beijing to consider calling off the rest of the torch's 136,788-km (85,000-mile) journey.
Dalai Lama to speak
Mr Obama said President Bush should boycott the opening ceremony if China failed to take steps to improve human rights in Tibet or help to end the alleged genocide in Darfur, in Sudan.
'Houdini torch' relay disappoints
"A boycott of the opening ceremonies should be firmly on the table but this decision should be made closer to the Games [in August]," he said in a statement.
Speaking on Tuesday, Mrs Clinton said Mr Bush should not attend the ceremony without "major changes by the Chinese government" over Tibet and Darfur.
The US House of Representatives has overwhelmingly passed a motion condemning China's "extreme" response to protests in Tibet.
Mr Bush himself called on the Chinese government to begin a dialogue with the Tibetan spiritual leader, the Dalai Lama.
The Dalai Lama, who is due in Tokyo en route to the US, is expected to give his reaction to the torch protests at a news conference shortly.
A spokeswoman for UK Prime Minister Gordon Brown said he would not be attending the opening ceremonies, and had never intended to do so. However, he will be at the closing ceremony.
Whisked away
Thousands of spectators had been waiting hours to see the torch pass through San Francisco and demonstrators were out in force along the waterfront relay route.
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But immediately after the torch was lit, the torch-bearers turned into a warehouse building, disappearing for an hour.
They reappeared at a new starting-point across the city where it was handed to two runners, away from the protesters.
The planned waterfront closing ceremony in Justin Herman Plaza was moved to a motorway fly-over.
"We assessed the situation and felt that we could not secure the torch and protect the protesters and supporters to the degree that we wished," San Francisco Mayor Gavin Newsom told Reuters news agency.
In Paris, the torch had to be extinguished three times, while in London there were 37 arrests.
OLYMPIC TORCH ROUTE
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