She said stigma was cruel and irrational.

"Stigma makes it easier for political leaders to stand in the way of saving lives," Melinda Gates said. "In some countries with widespread AIDS epidemics, leaders have declared the distribution of condoms immoral, ineffective, or both. ...But withholding condoms does not mean fewer people have sex; it means fewer people have safe sex, and more people die."

The United States is often harshly criticized for not doing enough to help poorer countries fight AIDS or demand U.S. drug companies to lower the cost of AIDS drugs. But Gates praised U.S. President George W. Bush for his pledge of US$15 billion (euro12 billion) over five years to combat the disease in 15 countries in Africa, Asia and the Caribbean, noting it was the largest single pledge ever made to fight a disease.

Other opening ceremony speakers included actor Richard Gere and pop singer Alicia Keys, who marched across the city earlier with several hundred African and Canadian grandmothers to show their support for the elderly women in sub-Saharan Africa forced to take care of their grandchildren after their own children had died.

It is estimated that 13 million children in sub-Saharan Africa have been orphaned by AIDS; more than all the children under 18 in Canada, Norway, Sweden, Denmark and Ireland combined.

Also attending the conference are former U.S. President Bill Clinton, as well as international AIDS experts such as Dr. Peter Piot, founder and executive director of UNAIDS and Stephen Lewis, the U.N. envoy for HIV/AIDS in Africa.

Piot, a microbiologist who launched the first international project on AIDS in the Democratic Republic of Congo in the 1980s told a youth congress earlier in the day that older men hold the key to ending the scourge.

"The toughest job in HIV prevention that we have is to make older men change their behavior," Piot said, referring to men who have multiple sex partners, then pass along HIV to their wives, girlfriends or boyfriends through unprotected sex.

"The future of this epidemic is in our hands," he said. "As long as we don't change our behavior and put girls and women at risk, again, it's not going to work."

This year marks the 25th anniversary of the first reported cases of human immunodeficiency virus, or HIV. Since the beginning of the pandemic, nearly 65 million people have been infected with HIV and AIDS has killed more than 25 million people.

Today, an estimated 40 million people are living with HIV, some 7 million more than the entire population of conference host, Canada.