PARIS, SEPT. 3 -- French President Francois Mitterrand today urged industrialized countries to renew pledges of aid to the world's poorest countries and to cancel more than $42 billion of their debts to help them avert potential catastrophe because of higher oil prices resulting from the Persian Gulf crisis.

The appeal by the French head of state was echoed by U.N. Secretary General Javier Perez de Cuellar, who warned that the perennial problems of hunger and overpopulation now plaguing a tenth of the world's population will soon be exacerbated by the jump in global oil prices since Iraq invaded Kuwait a month ago. Prices increased from around $18 a barrel before the invasion to as high as $32 a barrel the third week of August. Responding to the possibility of a diplomatic settlement of the crisis, prices toward the end of the month hovered around $25 a barrel.

The United Nations Conference on Trade and Development (UNCTAD), which will meet here over the next 12 days, was convened to draw attention to the worsening plight of the most impoverished habitants of the globe. The first conference, in 1981, identified 31 countries whose debt burdens, meager incomes and high rates of illiteracy and infant mortality were so appalling that the United Nations concluded that they required emergency assistance. Since then, the number of nations fitting such criteria has grown to 41 countries, comprising 440 million people.

Over the past nine years, few donor nations adhered to previous vows of higher aid levels. U.N. officials say that at least $36 billion a year in extra aid is now necessary to help the worst-off nations pull themselves out of the depths of excruciating poverty.

Their troubles will soon be magnified with the leap in oil prices. U.N. officials estimate that if oil prices remain at around $25 a barrel, the added burden for poor countries this year will be nearly half a billion dollars, which none of them is capable of paying.

Western delegates to the conference observed that donor countries are even less prepared than in 1981 to undertake new commitments because of gathering signs of a global recession, promises of substantial new aid to the Soviet Union and East European countries and now the anticipation of higher oil costs in the wake of the gulf crisis.

At the UNCTAD session nine years ago, donor nations agreed to set new aid commitments at 0.15 percent of their gross national products. But few abided by those promises, and in the case of the United States, aid to some of the poorest countries actually has shrunk.

Mitterrand today promised that France would surpass the previous goal and devote in the future 0.20 percent of its gross national product to help support the economies of the world's poorest nations. He said he will seek parliamentary approval to cancel debts and to make all further aid to poor nations in the form of outright grants, not loans.

In the past year, France has forgiven the debts owed by 35 African countries, many of them former French colonies. The cancelled sums of principal and interest have amounted to about $5 billion.

The French leader called on other industrialized nations to follow suit, but most nations have shied away from that kind of commitment because of concern about setting a precedent that would trigger demands by less desperate countries in the Third World to abolish their debts with richer nations.

Mitterrand pleaded that it is time for industrialized countries to take their earlier donation pledges seriously. He suggested that within the next five years they should agree to match the original UNCTAD goal of setting aside 0.15 percent of their GNP to aid the planet's poorest countries.

Perez de Cuellar urged the donor countries not to lose sight of the fact that a safety net is increasingly necessary because the growing interdependence of the global economy means that the collapse of some impoverished countries could have a serious impact in the developed world.

Mitterrand also called on the International Monetary Fund and the World Bank to examine ways, at their annual meeting this month, to soften the impact of higher oil prices and the U.N. embargo against trade with Iraq and Kuwait that has hit several poor countries, notably Turkey, Egypt and Jordan.

With delegations from both Iraq and occupied Kuwait at the conference, Mitterrand linked the search for a diplomatic solution to the gulf crisis with the fate of developing nations by stressing that "one of the best guarantees" for stability and growth in poor countries is "respect for borders."

"If it is necessary to correct those frontiers drawn in another era, why not work through international law and especially the United Nations to bring these changes about?" he asked.

Perez de Cuellar flew here Sunday from Amman, Jordan, after failing to make headway toward a diplomatic solution of the gulf crisis in talks with Iraqi Foreign Minister Tariq Aziz. He said he was disappointed by his discussions and said the Iraqis evinced no signs of willingness to withdraw from Kuwait.

Both he and Mitterrand were expected to meet tonight with Jordan's King Hussein to discuss diplomatic moves in the gulf crisis.