One of the largest conferences in the 27-year history of AIDS was set to open here Sunday with an expected turnout of 22,000 scientists, policymakers and grassroots workers.

The International AIDS Conference, held every two years, runs in the Mexican capital until Friday, and coincides with a relative lull in the long fight against the disease.

The theme, "Universal Action NOW," reflects an appeal to political leaders to maintain the momentum that began to build in mid-decade and has transformed access to precious antiretroviral drugs in poor countries.

A ceremonial concert late Sunday was to give the official kickoff, but workshops, seminars and other activities began several days before the start.

It is the first international AIDS conference to take place in Latin America.

Several thousand activists took part in a march on Saturday to protest AIDS discrimination that is widespread in this continent.

VIPs include former president Bill Clinton, a key figure in the campaign to slash the price of anti-HIV drugs to developing countries which are home to 90 percent of the 33 million people with the human immunodeficiency virus (HIV).

Insiders said they did not expect any breakthrough announcement in the arena of drugs, and braced for confirmation that the quest for a vaccine and an HIV-thwarting vaginal gel was mired in setbacks.

More positively, though, evidence has emerged that male circumcision can help prevent HIV infection among men -- a finding of great significance in southern Africa, the epicenter of the pandemic.

In a new report published on the eve of the conference, American health watchdogs acknowledged they had substantially underestimated the number of new HIV infections in the United States.

About 56,300 people were infected with the virus that causes AIDS in 2006, a figure 40 percent higher than the previous estimate of 40,000 new infections a year, the US Centers for Disease Control and Prevention (CDC) said in a study.

Gays, bisexuals and African-Americans account for most of the increase, it said.

A major theme at the conference will be access to anti-HIV drugs that have turned HIV from a death sentence to a manageable disease.

Thanks to a major increase in funding and cuts in the price of first-generation antiretrovirals, nearly three million needy people in developing countries have access to the lifeline drugs.

The triple "cocktail" rolls back the virus, thus helping to restore the immune defenses, but does not completely eradicate the pathogen.

"There has been a spectacular advance, but we are still very short of the mark," Jean-Francois Delfraissy, head of France's National Agency for AIDS Research (ANRS), told AFP.

"One of the tasks of the conference is to address the fact that there are three million people who now get the drugs, but another nine million who do not."

According to UN agency UNAIDS, around 10 billion dollars was spent last year fighting AIDS in poor countries, but this was 8.1 billion dollars short of what was needed.

Simply to maintain the current pace of drug access will require funding to rise by 50 percent by 2010. Even more will be needed to meet the goal of universal access, set for that year, by the UN General Assembly.

New guidelines, issued by US experts on Sunday, said that people with HIV should ideally be treated far sooner than is generally advised.

Using antiretrovirals before the AIDS virus has made major inroads into the immune system appears to the reduce the risk of certain kinds of cancer and cardiovascular disease, according to the recommendations.

The guidelines are appropriate for developed countries, where the choice of drugs is wider and laboratory infrastructure to monitor patient health is better, said the International AIDS Society (IAS) panel.

More than 25 million people have died from AIDS since the immune-wrecking disease emerged in 1981 and 33 million today have HIV.