Film director Steven Spielberg and actress Mia Farrow joined activists worldwide Tuesday in using the Olympics as a backdrop to address human rights concerns, urging Beijing to exert political leverage on Sudan's government to help end the crisis in Darfur.

Spielberg announced he would no longer act as an artistic adviser for the opening and closing ceremonies, saying he could not reconcile working on the Olympics while China and other nations are not doing enough to ease the suffering.

"Sudan's government bears the bulk of the responsibility for these ongoing crimes but the international community, and particularly China, should be doing more," Spielberg said in a statement. "China's economic, military and diplomatic ties to the government of Sudan continue to provide it with the opportunity and obligation to press for change."

Farrow joined former Canadian Olympic swimmers Shannon Shakespeare and Nikki Dryden in delivering an open letter addressed to Chinese President Hu Jintao at the Chinese mission to the United Nations, condemning Beijing's support of the Sudanese government.

The letter was part of a "global day of action" that focused on Darfur, where more than 200,000 have died and an estimated 2.5 million have been displaced since 2003.

"We are all aware of the tremendous potential for China to help bring an end to the conflict in Darfur," said the letter, signed by Nobel Peace Prize laureates, celebrities and 13 former Olympians.

Farrow and several humanitarian groups had been critical of Spielberg for continuing to work on the Games, as the Sudanese military said it bombed three towns in west Darfur last week.

Spielberg, who sent a letter to Hu in April urging China to take a stronger stance against Sudan, still hopes to attend the Olympics. But he said his conscience is pointing him toward spending his time and energy on the relief effort instead of the ceremonies.

"The situation has never been more precarious," he said. "While China's representatives have conveyed to me that they are working to end the terrible tragedy in Darfur, the grim realities of the suffering continue unabated."

Several nations have attempted to put limitations on athletes who might try to use the Games as a platform for discussing Darfur.

On Monday, the British Olympic Association acknowledged that its team agreement appeared to go beyond rules laid out by the International Olympic Committee barring any "demonstration or political, religious or racial propaganda" at an Olympic venue or area.

The U.S. Olympic Committee plans to ask American athletes to comply with IOC rules, but won't impose additional measures.