On the sides of the dilapidated public buses that career perilously through the streets of Cairo, bursting to the seams with humanity, one can see a particularly relevant new ad these days -- for Tops condoms.

Ads for other contraceptives on billboards and light poles line the boulevard from the airport to downtown Cairo and appear regularly now on television and in the press.

The blitz campaign to popularize birth control and family planning is the work of a still little-known private organization called Family of the Future. Less than five years old, it is now leading the family planning effort here, which for decades has gotten nowhere because of a lack of government clinics and commitment and the opposition of conservative religious elements in Egyptian society.

Last year, Family of the Future, which is financed largely by the U.S. government, distributed enough contraceptives -- condoms, pills and IUDs -- to provide for 420,000 "couple-years of protection" -- the protection of one couple for one year. By comparison, the figure for the Health Ministry clinics was 380,000.

The history of family planning in Egypt tells a lot about how things do or do not get done in this country, and about the difficulties the government has in dealing with its worst problems.

The population explosion is widely regarded among many western experts -- and now by President Hosni Mubarak, too -- as the most serious underlying social and economic issue facing Egypt. Last March, Mubarak told a conference bluntly that if birth control was not pursued in earnest, "we will have terrible famine, unemployment and terrorism."

Every month, there are 100,000 more mouths to feed in Egypt -- 1.2 million a year. The population has reached at least 48 million, and some estimates put it at 50 million.

The population growth rate was 2.76 percent in 1983 while the increase in food production stood at only 2 percent. Every year, the "food gap" widens and Egypt becomes more dependent on imports to feed itself.

The growth rate has come down only slightly from 1981, when it stood at 2.82 percent, but it is still higher than it was 10 years ago, according to the U.S. Agency for International Development's population office here.

Today, Egypt spends $8 million daily on food imports, but it spends less than that amount yearly on birth control.

This desert country, with only 2.5 percent of its land cultivated and the sliver of green along the Nile steadily shrinking, is being smothered slowly under the weight of its own humanity. Already, 2 million to 3 million Egyptians have gone abroad for jobs and space.

To proponents of population control, Egypt is a prime example of a government and society refusing to deal with its problems. "Economists here simply don't think it is a problem," said one frustrated western population expert.

In the past, fitful efforts by the few Egyptians and outsiders who did recognize it as a problem failed largely because the state insisted on dealing with family planning but had neither the commitment nor the means to do so.

"It's a critical issue, whether it's the state's job or done through other means," another western expert said.

In the absence of a clear response by the state on the matter, Family of the Future has stepped in, with the support of AID, which finances most of the budget and provides most of the supplies.

Its executive director, Effat Ramadan, explained in an interview at the organization's headquarters in Dokki, a Cairo suburb, that its approach differed radically from the state's.

"We apply all commercial marketing techniques to the social issues of family planning," he said. "We tell private physicians and pharmacies about different means and ways, and use social promotion, advertisements in clubs, factories, fairs and on TV, radio, billboards and light poles."

Family of the Future, which has a staff of 200 and four regional offices, is now providing contraceptives to 4,000 pharmacies and 4,000 doctors and covering all urban areas of the country, according to Ramadan. AID gave $2.6 million last year plus most of the contraceptives, and hopes to provide most of this year's $5 million budget.

Ramadan attributes part of his group's success to Mubarak's quiet backing for the "private way" of tackling the problem.

"The president is supporting private efforts because he feels people will listen more and because we are closer to the community," he said.

"Private organizations are more aggressive and direct. We are able to make decisions on the spot without 10,000 committees," he said.

Another factor in Family of the Future's success has been the blessing given to its program by Sheik Mohammed Metwali Sharawi, Egypt's most popular Moslem leader.

"He's against abortion and sterilization but not against 'temporary family planning methods,' " Ramadan said. "He's talking about it on TV and in his book."

In its sales pitch, Family of the Future carefully has avoided the term "birth control," because, Ramadan said, "this is very much rejected in a Moslem country." In past programs it stirred the anger of Moslem fundamentalists, and their potential opposition remains a crucial factor to the success of the current one. Fundamentalists in the lawyers' union have gone to court to seek a ban on the contraceptive ads on buses and television, arguing that they are in bad taste in a Moslem society.

Ramadan said his objective is to get the population growth rate down to between 2 and 2.2 percent -- meaning an average family size of four to five persons. But he said he realizes this cannot be done until the family planning program reaches the countryside, where half of Egypt's population lives. In Upper Egypt, in the south, the fertility rate still averages 6.3 children per woman in a liftime compared with 3.5 in Cairo and Alexandria.

Now, after nearly three years of discussion and bureaucratic infighting, the government is about to launch its own campaign. A revitalized National Family Planning Council, with Mubarak as president and the prime minister as the number two official, is scheduled to begin work within weeks.

The council's actual program has not been announced, but western experts do not expect a "Chinese approach" of social coercion.

In the view of one expert, the Egyptian government, after three decades, finally may have discovered that "the need for contraceptives is not much different from the need for aspirins. All you have to do is make them available."