Predicting "unimaginable human misery" in sub-Saharan Africa unless international lenders and African governments change their policies, former World Bank president Robert S. McNamara yesterday called for a 30 percent increase in aid from other nations and a quadrupling of loans by the World Bank.

Years of drought and famine have obscured the broader structural problems that have made African nations dependent on imports of food, caused population to soar and devastated grazing and farm land, McNamara said.

"The harsh truth is that sub-Saharan Africa today faces a crisis of unprecedented proportions," McNamara said in remarks prepared for the Consultative Group on Agricultural Research. "The physical environment is deteriorating. Per-capita production of food grains is falling. Population growth rates are the highest in the world and rising. National economies are in disarray. And international assistance in real terms is moving sharply downward."

Speaking to reporters before the evening speech, McNamara credited Treasury Secretary James A. Baker III with bringing about "a dramatic change of attitude" within the Reagan administration on aid to developing nations.

Baker's active approach to Third World debt, laid out at an international meeting in Seoul last month, "was a very wise, perceptive, courageous statement," McNamara said.

But he noted that Baker had not mentioned the needs of African nations when he laid out the new program, and conceded the secretary's push for increased lending by commercial banks and international lending agencies "went only part way."

It will be "hard as hell" to raise nation-to-nation aid from the current $5 billion to the $6.5 billion he thinks is necessary for Africa, McNamara said.

McNamara cited the rate of population increase in many African nations -- the fertility rate in Kenya, for example, is eight children per woman -- as "the most important long-term issue" those countries face, a "ticking time bomb" that will double the population of the sub-Saharan nations in 22 years and quadruple it in 44 years.

He called for improved family planning programs to halt what is now a 3.2 percent rate of population increase in the sub-Saharan region, the highest rate in the world.

Although McNamara called efforts by the Reagan administration to cut funding for international population programs "disgraceful," he said the impetus for population control must come from the countries themselves.

He also suggested a special research program to be established under the auspices of the World Bank to find ways to reverse the erosion and shrinkage of forests, farm land and grazing land.

But all these measures will fail, McNamara said, unless African governments themselves recognize the problems and change some of the policies that cause them.

If they do not encourage more exports by adjusting exchange rates, reduce government spending, curb high inflation rates and cut intervention in the economy, adjustments will be more difficult to make later, he said.

"African leaders should not ignore the fact that there is increasing concern in the industrialized nations over a number of disturbing issues affecting many African countries," McNamara said. "There is concern over the pervasiveness of corruption. There is concern over the use of scarce resources to build large defense establishments and luxury projects. There is concern over harsh treatment of regional groups. And there is concern over the repression of internal dissent."