U.N. Secretary General Kofi Annan urged stronger leadership from the grass-roots level to the heads of government to reverse the global AIDS epidemic as he opened the 15th international conference on the disease Sunday night.

"AIDS is far more than a health crisis," Annan told a crowd of more than 17,000 cheering delegates. "It is a threat to development itself."

Annan said it was appropriate that the conference was being held in Asia, where one in four infections in the world occurred last year. "There is no time to lose if we are to prevent the epidemic in Asia from spinning out of control," he said.

Three years ago, the United Nations, during the first General Assembly session devoted to a disease, pledged to deliver the money and action needed to beat AIDS, he recalled.

Progress has been made, he said. "Significant new resources have been pledged" through the Global Fund to Fight AIDS, Tuberculosis and Malaria, he said, and most countries have adopted strategies to tackle the disease, "yet we are not doing nearly well enough."

The world is falling behind in reducing the scale and impact of the epidemic, he said, referring to the World Health Organization's goal of helping governments and organizations put 3 million people in poor countries on life-prolonging antiretroviral drugs by 2005. A U.N. report issued before the conference said 38 million people worldwide were estimated last year to have HIV, the virus that causes AIDS.

A top priority must be the training and recruitment of health care workers to support treatment and prevention programs, Annan said, including ensuring that infected health care workers have access to treatment. In many hard-hit countries, he said, "AIDS drives a cruel and vicious circle by striking at those who are most badly needed to fight the epidemic."

Annan noted that women account for about half of all adult infections; in sub-Saharan Africa they make up about 58 percent of cases. Among people younger than 24, girls and women make up nearly two-thirds of those living with HIV, he said.

Stronger leadership at every level is needed, he said, to stem the pandemic, which has killed 20 million people since 1981. "There must be no more sticking heads in the sand, no more embarrassment, no more hiding behind a veil of apathy," Annan said.

The meeting is taking place against a backdrop of tension over the role of the United States, the wealthiest and most powerful country, in the global AIDS fight. This year, the U.S. government sent one-quarter as many people to the conference as it did to the meeting two years ago in Barcelona. At that event, activists heckled Health and Human Services Secretary Tommy G. Thompson. U.S. officials said the reduced delegation this year was a cost-saving move, not a snub.

At a news conference Sunday afternoon, Joep Lange, president of the International AIDS Society, which is co-chairing the conference, called the slimming-down of the U.S. delegation "shameful" and "tragic." He lamented that "people in the field who have to do the job are directly prevented from coming here."

During a morning news conference convened to highlight research published in the Journal of the American Medical Association, JAMA editor Catherine D. DeAngelis said a scientist from the Centers for Disease Control and Prevention who had co-written a paper on preventing HIV transmission from mothers to newborns was not allowed to attend. The association had offered to pay the scientist's way, DeAngelis said.

Ronald O. Valdiserri, head of the CDC delegation, said he had offered to send a scientist from the agency who was not part of the research team to the news conference. JAMA officials declined, and instead had a co-author, Mardge H. Cohen, of Cook County Hospital in Chicago, present the findings, which will appear in this week's issue of the journal.

About 1,000 activists marched outside the convention hall Sunday afternoon, urging increased access to cheaper drugs, condoms and clean needles. "Medication for all nations!" they shouted. "Bush tells lies, condom saves lives," read one placard.

During the opening ceremony, Thailand's prime minister, Thaksin Shinawatra, endorsed Annan's call for strengthened leadership. Thailand, which slashed HIV infections by about 86 percent from 1991 to 2003, has been hailed as a model for the rest of the world. Thaksin said Thailand was committed to universal access to antiretroviral drugs, with a target of reaching 50,000 people this year. He also said that his government would give excess drugs to Laos, Burma and Cambodia, countries bordering Thailand that face significant challenges in fighting the disease.

During part of Thaksin's address, activists booed and unfurled a large black banner saying "Thaksin Lies." Human rights activists have charged that an aggressive drug crackdown in Thailand had driven users underground, where they are harder to reach. More than 2,500 people have been killed in the country's war on drugs. Human rights groups say many of the deaths were extrajudicial killings, but the Thai government insists that most were the result of violence among drug dealers.

In other developments, Richard Feachem, executive director of the Global Fund to Fight AIDS, Tuberculosis and Malaria, urged the United States to rapidly approve generic drugs for poor nations.

Brown reported from Washington.

The 15th international conference on AIDS is being hosted by Thailand, where HIV infections decreased about 86 percent between 1991 and 2003. About 1,000 activists marched outside the 15th International AIDS Conference in Bangkok, urging broader access to cheaper drugs.