African grandmothers charged late in life with the care of AIDS orphans, Hollywood actors, altruistic youth and seasoned veterans in the battle against the disease that has killed 25 million people gathered Sunday to kick off a weeklong conference to share research, tell stories and renew their commitment to halt the spread of the virulent virus.

More than 24,000 scientists, activists, celebrities, HIV-positive people and humanitarians helping those cope with the devastating virus were to attend an opening ceremony with a keynote address by Bill and Melinda Gates and entertainment by Alicia Keys, Richard Gere and other celebrities.

The Gates told a news conference before the opening ceremony that they would be calling on world leaders to accelerating the search for microbicides and other new HIV prevention tools.

"Obviously the AIDS epidemic is going to require all actors, particularly governments, to dig deep and make this a high budgetary priority," Bill Gates said.

Gates, who recently announced he would step down from his day-to-day duties at Microsoft and devote more time to the philanthropy of the Bill & Melinda Gates Foundation, said the search for a vaccine to prevent AIDS was now the foundation's top priority.

He believes the discovery of a microbicide or oral prevention drug to reduce HIV transmission could be "the next big breakthrough" in the fight against AIDS, particularly for women in the Third World.

"A woman should never need her partner's permission to save her own life," Gates said. "There's progress on these, but the pace has been too slow."

Actor Richard Gere, former U.S. President Bill Clinton and international AIDS experts such as Stephen Lewis, the United Nation's special envoy on HIV/AIDS in Africa, and Dr. Peter Piot, executive director of UNAIDS, were also attending the 16th International AIDS Conference.

Piot, a microbiologist who launched the first international project on AIDS in the Democratic Republic of Congo in the 1980s, told a youth congress that while they were at greater risk of contracting the disease, 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 told some 1,000 youth delegates, referring to men who have multiple sex partners then pass along HIV to their wives, girlfriends or boyfriends through unprotected sex.

Though one half of the world's new cases of HIV are now among people younger than 25, Piot said, "The future of this epidemic is in our hands, it's in the hands of boys and men, whether it's men who have sex with men or men who have sex with women. 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 worldwide 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.

There are still an estimated 11,000 new HIV infections and 8,000 deaths every day, most of them in sub-Saharan Africa, where nearly 64 percent of those infected worldwide live today.

Dozens of grandmothers from Africa who now are rearing their grandchildren after the deaths of their children to AIDS, marched through the streets of Toronto on Sunday with their Canadian counterparts, singing and dancing with pop star Alicia Keys.

The Stephen Lewis Foundation estimates that 13 million children in sub-Saharan Africa have been orphaned by AIDS; more than all the boys and girls under 18 in Canada, Norway, Sweden, Denmark and Ireland combined.

Some of the Canadian grannies came with walkers, others wore running shoes. Many of the African grandmothers sported headdresses.

Grandmothers are at the heart of the response to the AIDS pandemic in Africa as they bury their own children and then begin parenting their grandchildren.

"The grandmothers are the silent victims of this pandemic and it's now I feel the silence has been broken and that it's the time to be heard," said Keys.

"We grandmothers deserve hope. Our children, like all children, deserve a future," said Kenyan grandmother Joyce Gichuna.