Swaziland, with the world's highest rate of HIV, cut Aids day events, and South Africa's health minister publicly refused to back anti-retroviral drugs.
US President George W Bush pledged new funds and called for decisive action. The EU stressed the need for effective measures to prevent the disease.
More than 40m people are infected with HIV/Aids, according to the UN.
"The lessons of nearly 25 years into the Aids epidemic are clear. Investments made in HIV prevention break the cycle of new infections," said Peter Piot, executive director of UNAids.
"By making these investments, each and every country can reverse the spread of Aids."