The American Health Secretary, Tommy Thompson, has warned that the world is losing the fight against Aids.
An estimated 40m people are infected with HIV worldwide
Speaking in Zambia on World Aids Day, Mr Thompson called on the international community to intensify its efforts to combat the disease.
To mark the day, the United Nations unveiled ambitious plans to supply three million HIV sufferers with the drugs they need by the end of 2005.
An estimated 40 million people are now infected with HIV around the world.
According to the UN's World Health Organization (WHO), six million people in developing countries need anti-retroviral drugs, but less than 300,000 actually receive it.
The new strategy, called the "3x5" plan, will make cheaper drugs more available and simplify treatment regimens for millions of HIV sufferers as part of a two-year plan costing $5.5bn.
Launching the campaign in the Kenyan capital, Nairobi, WHO assistant-director General Jack Chow said HIV/Aids had become "the premier disease of mass destruction".
"It is inexorably converting developing nations into Aids-imploding nations."
In Zambia, one of the worst-hit nations, the US health secretary appealed for a redoubling of efforts to fight the disease, which is being carried by an estimated 26.6 million people in sub-Saharan Africa alone.
"We appear to be losing the fight against Aids at the moment," said Mr Thompson.
"We need America, the European Union and everybody. Nobody is going to be spared unless we all come together in the fight against this disease."
World leaders and millions of people around the world have highlighted the day with a series of events and planned activities.
- Former South African President Nelson Mandela urged the world to fight against the stigma associated with HIV, warning that "Many will die because of feeling less than human"
- Chinese Prime Minister Wen Jiabao made an unprecedented visit to people with Aids in a Beijing hospital
- UK Prime Minister Tony Blair said in a newspaper article the world had a "moral duty" to unite to fight Aids
- The Vatican issued a five-page statement, defending its opposition to condoms and stressing the importance of "fidelity, chastity and abstinence"
- In a rare admission, Saudi Arabia said there are nearly 7,000 Aids cases in the kingdom and announced a national strategy to combat the disease
- In Singapore, scantily-clad women handed out free condoms
- Sports stars and actors spearheaded an Aids awareness campaign in India, while thousands of students joined a "walk for life" rally in Delhi
- Hundreds of gay men and their families held a march in the Indian city of Bombay (Mumbai), urging the government to repeal a law banning sex between members of the same gender
- Some of Thailand's leading pop stars staged a free concert in Bangkok to raise Aids awareness
- In Cambodia, thousands of people sporting T-shirts with Aids slogans attended an Aids rally in the capital, Phnom Penh, before taking buses to the countryside to spread anti-Aids messages
- In Malaysia, the daughter of former Prime Minister Mahathir Mohamad, launched an Aids exhibition, warning about an alarming "level of denial" of the disease in her country
- In Albania, which has registered little more than 100 HIV cases in the past 10 years, high school students marched through the capital with candles and a banner reading "Protect Yourself and Others"
- In Europe, planned events included candlelight vigils in British cities and the Swedish capital, Stockholm, and workshops and rallies in Turkey and Portugal
- Activities were also planned across the Caribbean and Latin America, which hosts a world Aids conference for the first time on Tuesday
Last week's report from UNAids and the WHO warned that the disease was still spreading.
It said many countries were on the verge of fresh epidemics.
The number of reported infections is rising sharply in China, India, Indonesia and Russia, mostly due to HIV transmission through injecting drugs and unsafe sex.
The 3x5 campaign is being spearheaded by three United Nations agencies - the WHO, UNAids and the Global Fund to fight Aids, TB and Malaria.
Peter Piot, executive director of UNAids, said ensuring people have access to drugs is the only way to fight the disease, which he called a "global emergency".
He said only 75,000 HIV-infected Africans were getting treatment - out of the four million who need it.
"That is really not acceptable and we have no chance of halting this epidemic if we're not going to make sure that everybody who needs it has access to treatment."
UN Secretary General Kofi Annan warned that targets to fight HIV/Aids spelt out in the 2001 Declaration of Commitment are being missed.
"The epidemic continues its lethal march around the world, with few signs of slowing down," he said.
"We must work even harder to match our commitment with the necessary resources and action."