Outgoing US President George W. Bush marked the occasion by highlighting his efforts to battle the deadly disease, particularly in Africa.
He announced that his President's Emergency Plan for AIDS Relief, or "PEPFAR," had already met its goal of helping to treat two million people living with HIV/AIDS by the end of 2008.
"PEPFAR is bringing hope and healing to people around the world. On our trips to Africa, Laura and I have witnessed first hand the gratitude of the African people," Bush said.
The White House says that just 50,000 people in all of sub-Saharan Africa were receiving life-saving anti-retroviral treatment when the program was born in 2003.
In South Africa, the country with the highest number of sufferers in the world, the government was mapping out its AIDS strategy under a new health minister as part of a sea-change in attitudes.
South Africans held a moment of silence at midday as a mark of respect for victims of the virus which has affected some 5.5 million people.
Newly appointed Health Minister Barbara Hogan Monday promised to "urgently scale up" mother-to-child prevention programmes and urged men to test for HIV, the virus that can lead to full-blown AIDS.
"We encourage all men, I repeat all men, to test themselves for HIV to protect themselves and the people they love," Hogan said. "We all know that together we shall overcome."
Celebrities also used their media-drawing power to raise AIDS awareness with France's first lady Carla Bruni-Sarkozy announcing that she had accepted a new mission as ambassador for the Global Fund to Fight AIDS, Tuberculosis and Malaria.
Through field trips, advocacy and by mobilising other celebrities to add their voice to the struggle against AIDS, Bruni-Sarkozy hopes to bring renewed focus to a cause that appears to be suffering from donor fatigue.
In Beijing, Hu's visit to a hospital was also designed to strip away some of the stigma attached to the virus and the Chinese leader praised volunteers as an "indispensable force" in the battle against the disease.
"One of the important tasks of volunteers is to spread knowledge about AIDS prevention so that every citizen can have that knowledge," Hu said in a state television report.
"This way, all of society can work together to prevent AIDS."
China along with the United Nations launched a campaign Sunday to raise awareness about HIV/AIDS. The world's most populous country has about 700,000 people who are HIV-positive, according to a previously released estimate by the Chinese government and UN health organisations.
However, only about 260,000 have been officially identified as having AIDS.
Other countries also released their AIDS statistics, including Iran which estimated that the Islamic republic has 80,000 HIV cases, four times more than the number of people registered, the state news agency IRNA reported.
In Mynamar, UNICEF said there are approximately 240,000 people living with HIV, of which almost two thirds are under the age of 24.
Britain's Prime Minister Gordon Brown noted that while significant progress has been made in fighting the spread of AIDS, the impact of the disease "remains immense," especially in the poorest corners of the globe.
African nations have expressed concern that the world's richest countries grappling with the global economic crisis may cut back on AIDS funding.
But Brown urged world leaders "to hold firm to their promises to improve the health of the poorest, even in the midst of the current economic challenge."
UN Secretary-General Ban Ki-moon marked the event by announcing the appointment of Mali's Michel Sidibe of Mali as the head of the world body's AIDS-fighting agency.
Sidibe, 56, will start his new job as executive director of the UN AIDS program based in Geneva, Switzerland. He has served as UNAIDS deputy executive director since 2007 and replaces Belgian Peter Piot.