The opening ceremonies of the 16th International AIDS Conference in Toronto are underway, attended by thousands of researchers, politicians and activists.

Conference co-chairman Dr. Mark Wainberg was one of the first to speak and criticized the prime minister for missing the event.

"We are dismayed that the prime minister of Canada, Mr. Stephen Harper, is not here this evening," Wainberg said Sunday.

"The role of prime minister includes the responsibility to show leadership on the world stage. Your absence sends the message that you do not consider HIV/AIDS as a critical priority, and clearly all of us here disagree with you," he said.

Many in the audience stood up to cheer.

While Harper declined to attend the event, Health Minister Tony Clement was present to deliver a speech. But as he addressed the crowd, a group near the stage stood up and chanted "Where is Harper?"

Clement was preceded by Governor-General Michaelle Jean, who officially opened the conference and told the crowd: "Your efforts will ensure that one day soon, we will put an end to one of the most insidious epidemics of our time."

Billionaire Microsoft founder Bill Gates spoke before and during the ceremonies by calling on governments around the world to do more in the battle against HIV/AIDS.

"Obviously the AIDS epidemic is going to require all actors, particularly governments, to dig deep and make this a high budgetary priority," Gates said at an news conference before the opening ceremonies.

"The amount of money that's required for universal treatment or the things around prevention far exceed the amount that any individual government, certainly any foundation, can possibly provide."

The response was in answer to a reporter's question about how much money will be needed to fight the disease.

Gates and his wife Melinda, through their foundation, announced last week they are contributing $500 million over five years to fight AIDS.

"Our belief is that every life on this planet has equal value and we cannot turn our backs on anyone in this disease," said Melinda.

She called for more research on microbicides, saying they would allow women to protect themselves from the virus.

"Today, fewer than one in five people who are at high risk of HIV don't have access to things like condoms, clean needles, HIV testing, and education," said Melinda, adding stigma is a major issue in the epidemic.

Wainberg said those suffering from AIDS around the world need better access to drugs.

"Access to HIV drugs is a right and not a privilege -- at least that is our goal," he said.

"This conference plays such a vital role in combating the spread of HIV."

Wainberg said there will be scientific discussions about new drugs and new drug classes for people living with HIV throughout the six-day event, the largest of its kind.

"It is time now we move from crisis management to long-term sustainable response," said UNAIDS executive director Peter Piot.

Diagnosed with HIV at age 18, Frika Iskandar, from Indonesia, said she represents "the new face of AIDS -- a young Asian woman."

She said she was recently refused treatment by a dentist out of ignorance. "The stigma and discrimination is still there. Nothing has changed," Iskandar said.

About 24,000 delegates and 3,000 journalists have come to Toronto for the now-biennial meeting. The event begun 21 years ago.

The opening ceremonies are being held at Toronto's Rogers Centre, and among guests expected to attend are actor Richard Gere, the Gateses, former U.S. president Bill Clinton, the crown prince and princess of Norway, UN AIDS for Africa envoy Stephen Lewis, and actors Sandra Oh and Olympia Dukakis.

A concert featuring Alicia Keys, Barenaked Ladies, Our Lady Peace, Chantal Kreviazuk, the Blue Man Group and Amanda Marshall is to follow.

About 20,000 tickets have been offered to the general public for the conference that closes Friday, with proceeds going towards numerous AIDS-related groups.

AIDS has killed 25 million people in the last 25 years and infected about 40 million worldwide.

With files from The Canadian Press