Global health chiefs are to spell out plans to ensure three million people with HIV get the drugs they need by the end of 2005.
An estimated 40m people are infected with HIV worldwide
The campaign, being launched as part of World Aids Day on Monday, will target those in the world's poorest countries.
An estimated 40 million people are now infected with HIV around the world.
In a message to mark the day, a top health official said tackling Aids might be the toughest health assignment the world has faced.
"The lives of millions of people are at stake. This strategy
demands massive and unconventional efforts to make sure they stay alive," said World Health Organization Director General Dr Lee Jong-wook.
World leaders and millions of people around the world have highlighted the day with a series of events and planned activities.
- Chinese Prime Minister Wen Jiabao makes an unprecedented visit to people with Aids in a Beijing hospital
- UK Prime Minister Tony Blair says in a newspaper article the world had a "moral duty" to unite to fight Aids
- The Vatican issues a five-page statement, defending its opposition to condoms and stressing the importance of "fidelity, chastity and abstinence"
- In a rare admission, Saudi Arabia says there are nearly 7,000 Aids cases in the kingdom and announces a national strategy to combat the disease
- In Singapore, scantily-clad women hand out free condoms
- Sports stars and actors spearhead an Aids awareness campaign in India, while thousands of students join a "walk for life" rally in Delhi
- Hundreds of gay men and their families hold a march in the Indian city of Bombay (Mumbai), urging the government to repeal a law banning sex between members of the same gender
- In Cambodia, thousands of people sporting T-shirts with Aids slogans attend 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, launching an Aids exhibition, warns about an alarming "level of denial" of the disease in her country
- In Europe, planned events include candlelight vigils in British cities and the Swedish capital, Stockholm, and workshops and rallies in Turkey and Portugal
- Activities are also planned across the Caribbean and Latin America, which hosts a world Aids conference for the first time on Tuesday
Officials say the so-called "3 x 5" campaign - to provide anti-retroviral treatment to three million people with Aids by the end of 2005 - is vital if they are to save millions of lives and stop Aids from spreading.
Three out of four people who die with the disease live in sub-Saharan South Africa.
Around 14,000 people become infected with the disease every day. It has claimed the lives of three million this year.
Last week's report from UNAids and the World Health Organization 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 World Health Organization, 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."
Speaking at a World Aids Day commemoration in Botswana, Mr Piot said that despite having the highest adult HIV prevalence in the world, the government had mounted an effective response to the epidemic in recent years.
Richard Feachem, executive director of the Global Fund, hailed the campaign.
"The 3x5 campaign is the kick-start to that effort. It is the engine that will drive us forward and expand access to treatment very rapidly. The world desperately needs this."
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."