When 1,000 teenagers gathered in a Moscow movie theater Thursday to see a show, they heard a message that few Russians have heard: When you have sex, use a condom.


In observance of World AIDS Day, the children's organization Prospekt Mira held a variety show in southern Moscow, interspersed with brief messages about the AIDS virus and ways to prevent catching it.


This is the first year that Russian authorities have actively propagated condom use as a way to avoid contracting AIDS, overcoming a tradition of prudishness and, until recently, hardheaded denial of the existence of AIDS in the country. Even now, officials insist only 831 people are HIV positive in Russia, rejecting much higher estimates by AIDS experts.


Vadim Pokrovsky, head of a government AIDS research center, last week chided the government for spending $25 million on 24 million AIDS tests each year while allotting little for AIDS education. The most widely televised advertisement, broadcast in previous years, warned that AIDS kills but did not suggest the use of condoms.


Thursday's show went a step further.


Five teenagers, members of a group of 50 children who have been trained by American AIDS educators to give seminars in high schools, advised the audience, made up largely of children aged 13 to 15 years, to use condoms and clean needles and not to be afraid of people who have the disease.


Tanya, 14, said she had not known that babies can be born HIV positive if their mother is infected, and added that she had only heard vague talk about how to use protection during intercourse.


"We often don't know how to share such information with the children," said Natalya Yefimtsova, deputy director of School 575, who escorted a group of 50 children. "We still need to be educated."