SAN FRANCISCO — Quick treatment with the drug AZT in the earliest stages of AIDS infection can significantly slow the lethal disease, according to research presented Thursday at the Sixth International Conference on AIDS.

The study appears to provide the strongest evidence yet that early intervention can stall the emergence of AIDS in people infected with HIV, the AIDS virus.

“We have to come to the conclusion that the greatest advantage has to accrue with early intervention,” said Dr. Margaret Fischl. “It has to occur as early as possible in HIV infection.”

Fischl, a researcher at the University of Miami, pioneered the use of AZT in people who already have AIDS. However, most infected people are still outwardly healthy. So Fischl and other teams around the country have been exploring the risks and advantages of giving the medicine before AIDS infection reaches its final stages.

The conference opened Wednesday amid angry protests by thousands of demonstrators frustrated over the slow pace of progress in controlling the disease.

The demonstrations continued Thursday as several hundred protesters marched by government offices in San Francisco’s Civic Center. About 25 protesters were arrested after they blocked a busy intersection.

Another group rushed into the lobby of a building housing the state insurance commissioner, but left a short time later without incident.

Fischl’s recommendation is likely to be controversial. During the past year, some researchers have begun to question whether AZT, the only approved AIDS drug, should be reserved for use during the later stages of the disease.

The medicine interferes with the reproduction of HIV. When given to people with AIDS, the drug loses its punch within two or three years as the virus becomes resistant to it.

Some doctors believe that if given early in the disease, AZT will be powerless to slow the disease once AIDS develops.

Fischl’s conclusions were based on patients who were infected with HIV but had not developed any major symptoms of AIDS. Two hundred received placebos for a year and then switched to AZT for eight more months. Two hundred fifty-one others took AZT for the entire period.

She found that significantly fewer progressed to AIDS if they got the drug without delay.

Several other reports presented Thursday focused on how the infection is transmitted.

Dr. Nancy Padian of the University of California, San Francisco, found that women who have sex with AIDS-infected men are 12 times more likely to catch the virus than are men who are sexually exposed to infected women.

Her research, based on 58 male sex partners of infected women and 269 women partners of infected men, supports the widely held belief that the virus spreads more “efficiently” from men to women than from women to men.

During her study, only one man caught the virus from his female sex partner. This couple reported having intercourse without condoms more than 1,000 times over seven years.