In more than four years of conflict in Sudan's western region of Darfur, 200,000 people have died and 2.5 million have been driven from their homes, according to estimates from international experts. Khartoum says 9,000 people have died.
Sudan's government, in its largest offensive in months, attacked three towns in Darfur on Friday, forcing about 200,000 people from their homes and leading thousands to flee into neighbouring eastern Chad.
The United States pressed Sudan to stop the campaign.
U.S. actress Mia Farrow, who has led the coalition's global campaign to press China to change its policies, gathered a crowd outside the Chinese mission to the United Nations in New York as she tried to deliver the letter.
"China hopes that these games will be its post-Tiananmen Square coming out party. But how can Beijing host the Olympic Games at home and underwrite genocide in Darfur?" she said, stuffing the letter under the mission door after her knocks went unanswered.
The letter to Hu acknowledged Chinese support for a U.N. Security Council resolution calling for deployment of a U.N.-African Union peacekeeping force in Darfur.
"However, we note with dismay that the Chinese government worked to weaken the resolution before it passed," it said. The letter said China doubled its trade with Sudan in 2007 and continued its military relationship with the African country.
Jody Williams, a U.S. citizen who won the prize in 1997 for her campaign against land mines, said she and fellow female laureates had formed the Nobel Women's Initiative in 2006 to focus on conflicts and particularly their impact on women.
Mass rape has been a weapon of warfare in Darfur and in Myanmar, the former Burma, another Chinese-backed regime.
"In Darfur and in the case of Burma, China is the eight-jillion-ton elephant in the room and needs to use some of its weight in a positive way," Williams said by telephone from Virginia.
The Save Darfur Coalition said it staged similar events in Britain, Portugal and Italy on Tuesday and planned more protests in Nigeria, France, Australia and elsewhere.
The campaign has so far not called for a global boycott of the Beijing games, although activists advocate not attending or watching the Beijing Olympics on television.
China's Embassy had no immediate comment. But last month, the ruling Chinese Communist Party's flagship newspaper and foreign ministry said China would never submit to pressure from groups trying to use the Olympics to change Chinese policy.
