Homophobia and disappointments in the search for a vaccine were expected to dominate the 17th International AIDS Conference as an estimated 25,000 scientists, politicians, physicians and activists gathered in Mexico City.

UN Secretary General Ban Ki-moon told the conference on its opening day Sunday that discrimination against gays must end and called for countries to expand AIDS-prevention programmes for the high-risk group.

It is the first such AIDS conference in Latin America since the epidemic began in the 1980s. The gathering, expected to be the largest to date for the biennial conference, is to end Friday.

Mexican President Felipe Calderon, former Botswana president Festus Mogae and St Kitts and Nevis President Denzil Douglas each called for the end of discrimination against gay men.

Mogae was one of the first African leaders to publicly undergo an HIV test to set an example for his countrymen and -women.

Thousands of people demonstrated in the streets of the Mexican capital against homophobia. Gay men represent one-fourth of the new infections in Latin America.

Jorge Saavedra Lopez, the director general of Mexico's National Centre for Prevention and Control of HIV/AIDS and the country's first openly gay person to serve as a senior government official, is directly involved in the fight to reduce discrimination.

'It is difficult to evaluate the extent of homophobia in our country,' he said.

The world AIDS community received sobering news recently that a giant US public research programme was pulling the plug on the testing of a vaccine that had raised hopes at the conference two years ago in Toronto, Canada.

Dr Anthony Fauci, who heads the US National Institute of Allergy and Infectious Diseases; Seth Berkley, president of the New York-based International AIDS Vaccine Initiative; and other leading world researchers are to speak at the conference.

Some AIDS activists have called for the estimated 700 million dollars spent annually on research to be channeled into providing more of the antiretroviral drug therapy for disadvantaged populations, such as those in Africa.

On Friday, Latin American health ministers committed to a programme of expanded sex education for young people about preventive measures against HIV.

More than a quarter of a century since AIDS was first identified, 25 million people have died and an estimated 33.2 million people are currently living with HIV/AIDS.