LONDON, JAN. 28 -- Government delegations representing 95 percent of the world's population declared today that AIDS "poses a serious threat to humanity" and pledged to work for "urgent action by all governments and people" to combat the disease.

In a declaration at the close of a three-day conference here, representatives from nearly 150 countries, most of them government health ministers, said that public distribution of information and education about AIDS was the single most important thing to be done, in the absence of a vaccine or cure.

The meeting, jointly sponsored by the World Health Organization and the British government, was the first worldwide political gathering specifically on the topic of AIDS. In a series of short speeches and informal discussions, the health ministers shared statistics on the incidence of infection in their countries and the programs for public information, prevention and treatment they had developed.

Delegates also received an update on the latest international statistics on AIDS. Based on the number of people already believed to be infected with the AIDS virus, which can take 10 years or more to develop into the disease, WHO officials said that by 1991 AIDS will kill more young men below age 34 in developed countries than will the four current biggest killers -- traffic accidents, suicides, heart disease and cancer.

{U.S. Surgeon General C. Everett Koop said in an interview with The Associated Press that he has proposed mass, anonymous AIDS screening of all students at a major American university this spring, and possibly at a few urban high schools, to determine the incidence of the disease among young adults. He said no decision has been made on his proposal.}

According to WHO Director General Halfdan Mahler, the conference's most important contribution was that government and medical leaders had adopted the "revolutionary" concept that the spread of information can hinder the spread of disease by persuading people at risk to alter their behavior.

Until recently, Mahler said today, "information and communication has been to a large extent stonewalled by the health professionals, including myself. We now have to relearn that communications is decisive in fighting such a global threat as AIDS."

Tony Newton, the British health minister, said that the conference will help those government officials "who may have felt that they have been fighting a difficult battle in their own countries" to push for more political support.

Dr. Jonathan Mann, who heads the WHO special program on AIDS, said the most important thing about the declaration adopted today was its rejection of discrimination against those infected with the virus or suffering from the disease. At the same time, Mann and others at the conference repeatedly argued that there is no evidence that widespread screening for the disease, particularly on a mandatory basis, would contribute to stemming its spread.

Despite the agreement of all delegations to the final document, there were indications throughout the conference of remaining differences in assessing the threat of AIDS and deciding what to do about it.

Soviet Health Minister Yevgeny Chazov repeated the Soviet view that AIDS is a disease of western decadence. Nearly 98,000 foreign visitors had been screened for the virus, and 221 of them were found to be infected. At the same time, he said, more than 7,000 Soviet homosexuals and bisexuals had been tested, along with 43,000 people with multiple sexual contacts. So far, only one Soviet citizen had developed the disease, and 14 sexual contacts of this person had become infected.

But, in what was seen as some softening in the Soviet attitude toward homosexuality, which is outlawed there, Chazov said that "we should proceed from the fact that a growing number of AIDS victims and carriers of the virus will live among us."

Koop said there still are difficulties in communicating to the public in the United States, where tolerance is low for open discussion of sexual practices and it is still difficult even to mention the word "condom" on television.