Mexico City — What comes next to try to save third-world villages and city slums from an accelerating population growth that is threatening food supplies, overcrowding cities, and helping create widespread unemployment?
In the wake of the Mexico City population conference attended by 148 nations, with its last-minute political upheavals over the Middle East, delegates headed home saying that:
* The main target area must now be Africa, especially south of the Sahara. Africa has 537 million people and is expanding at a rate of 3 percent a year. The figure will double in the next 24 years, United Nations figures show.
''The 1960s saw Asia turn to family planning, and the Latin Americans did so in the 1970s,'' said Dr. Sheldon Segal of the Rockefeller Foundation.
''Yet African nations are not yet fully taking part.''
* Women must be given greater opportunity to learn and work outside the home.
''It is essential to involve women on an equal basis with men.... This necessarily involves equal access by women to education, training, and employment,'' the UN Secretary-General, Javier Perez de Cuellar, said here.
* Governments and private groups will try to make the most of the worldwide publicity generated by the conference. They will focus even more attention on reducing a global growth rate of more than 80 million people a year.
* Strengthened is an already-growing conviction of the third world that its3. 6 billion people are best served by economic growth linked to the spread of voluntary family-planning services.
* In the immediate post-Mexico City period, the US has at least temporarily lost its traditional role as world leader in family-planning ideas and support. New White House policies redirecting aid funds and stressing free-market economic growth are considered by the third world as out of step in the short term with poverty, famine, and overcrowding.
At least some of this leadership role is being taken over slowly by the World Bank, with a new effort, announced here, to at least double its financial aid and moral support in the next few years.
* The US Congress, led by Democrats in the House and Republicans in the Senate, is looking for ways to ensure that the world's largest private family-planning group does not lose $12 million in US aid money in fiscal 1985, which begins Oct. 1.
The group is the London-based, 103-nation International Planned Parenthood Federation (IPPF). In a US election year, the White House says it will cease to give aid to any such group that supports or promotes abortion abroad even with its own, non-US finances.
The IPPF spends about $200,000 in 10 countries where abortion is legal on broad or narrow grounds.
Rep. James H. Scheuer (D) of New York, here for a parliamentarians' meeting following the population conference, said Congress could force the White House to give IPPF its annual contribution of $12 million but would prefer to negotiate with the administration on the issue.
Meanwhile, the United Nations Fund for Population Activities will continue playing a large role. It has now received the last half of its fiscal 1984 US budget contribution ($19 million) after assuring the White House that it uses no funds to support abortion or coercion.
* On the other hand, right-to-life, anti-abortion groups plan a long-drawn-out campaign to whittle down US aid by criticizing US public and private groups and pushing for the right to audit their books.
The right-wing groups suspect US money still goes toward abortion policies abroad. The US government and private groups deny it.
* Governments can be expected to intervene more and more to promote family planning as an efficient method of lowering birthrates and eventually lifting per capita incomes.
The Chinese government is already trying to limit couples to having only one child. Singapore is attempting to influence family planning by public rewards and disincentives.
Lester Brown of the Washington-based Worldwatch Institute says other third-world governments may have to consider the same policy.
After traveling to 38 countries in 14 months to write a new book entitled ''The Crowded Earth,'' Pranay Guxte in New York concluded that the population ''time bomb'' has not been defused.
''Its fuse is long,'' he writes, ''and it is still burning.''
Mr. Perez de Cuellar told the conference here, ''The next 15 years are of crucial importance.'' He went on, ''A good beginning has been made, but it remains a beginning with unprecedented challenges still to be faced.''
US Rep. Sander Levin (D) of Michigan, a former US aid official, said here, ''The question is whether the new US policy switches will obscure the painstaking consensus that has developed over the past 10 years to link family planning and economic development.
''Well, the US policy has made some adjustments toward recognizing the importance of government aid. The whole thrust here at the conference, of all countries, is so strong that it will keep growing - and the US should not be in the caboose of this consensus.
''After all, we helped to create it.''
Representative Scheuer praised the ''unique'' role of the IPPF and pledged support.
If necessary, he said, he would work to direct the White House to provide it full aid funds by means of a continuing budget resolution later this year.