Leaders of 41 nations today expressed alarm about deteriorating world economic conditions and called for a "global approach" to the problem of relations between rich and poor countries.

The appeal in the final communique of the Commonwealth Heads of Government Meeting here was intended as a signal to the United States before this month's North-South summit in Cancun, Mexico, participants here said.

Concluding their eight-day meeting, the leaders of Britain and its former colonies endorsed several economic proposals viewed with skepticism, if not disapproval, by the Reagan administration.

Specifically, the Commonwealth leaders called for "global negotiations" under U.N. auspices on international economic development questions, establishment of a World Bank energy affiliate, conclusion of the Law of the Sea Convention and "immediate and effective action" on North-South issues.

However, the meeting also took account of President Reagan's free-market approach to economic issues, recognizing the role of private capital in the development of Third World countries.

On the political front, the conference participants, representing a quarter of the world's population, expressed concern about a "slide from detente to confrontation, mounting tension between the superpowers and the buildup of nuclear arms . . . ."

Among other measures, the Commonwealth called for the independence of Namibia from South Africa "as early as possible in 1982," demanded "full and effective implementation" of the arms embargo against South Africa, "condemned any attempt from any quarter to subvert the legitimate government of Angola," and called for the withdrawal of foreign forces from Afghanistan and Cambodia. In addition, the communique condemned any "foreign interference" in Poland and said "most" leaders at the conference agreed that the Palestine Liberation Organization "must be involved in negotiations" for a Middle East settlement.

Although the United States was not mentioned by name, conference sources said the reference to Angola reflected concern about the recent repeal by the U.S. Congress of the Clark Amendment, a measure that prohibited any aid to pro-Western rebels fighting Angola's Marxist government.

The communique also avoided mentioning the United States in the economic provisions that delegates said privately were meant as signals to Washington. In fact, the reaction of developing countries to what they consider Reagan's discouraging economic policies was surprisingly subdued, conference sources said.

This appeared to reflect the considerably weakened position in which developing countries find themselves vis-a-vis the Western industrialized countries going into the Cancun summit, which will take place Oct. 22 and 23.

Whereas six years ago developing countries were stridently insisting on a "new international economic order" in the so-called "North-South talks" with industrialized countries, the Melbourne conference was practically devoid of such demands. Instead, the Third World countries made repeated appeals for greater compassion, underscoring their cases with shocking statistics on their economic difficulties.

A background paper issued by the Commonwealth Secretariat, which largely reflects Third World views, stated, "The world economy is in its worst crisis since the Great Depression of the 1930s."

One of the leading exponents of Third World views here, Tanzanian President Julius Nyerere, said that 700 million to 800 million people were living in "absolute poverty."

He added, "none of us here suffers personally from absolute poverty. But to those of us whom history has condemned to the management of poverty, poverty is an agony all the same."

Nyerere sharply criticized the World Bank and the International Monetary Fund, calling for "a new Bretton Woods conference" to design a financial system better equipped to deal with current world economic problems.

However, several other leaders of developing countries expressed a minority view that echoed President Reagan's approach to economic development problems. They stressed adjustment to economic conditions and efforts to attract foreign private capital to promote growth.

"The developing world needs to stop groping in the antechambers of colonial history for solutions based on the cry that somebody owes us a living," said Jamaican Prime Minister Edward Seaga in a speech.

Instead, he said, developing states should seek "an honorable living based on producing what we can produce better than anyone else."

Despite such differences in approach, there was little disagreement on the extent of the problem.

"The gap between the rich and poor countries has grown wider than ever, and the least developed countries are in a grim struggle for their survival," said the prime minister of Bangladesh, Shah Azizur Rahman.

A major problem is that the oil shock of 1979 and 1980 has hurt developing countries much more severely than the 1973 oil crisis and has been compounded by the current high interest rates. As a result, balance of payments and debt service problems of oil-importing, developing countries have assumed staggering proportions.

The Commonwealth leaders said they noted the Brandt Commission report, which argued that industrialized countries stood to gain from Third World development and that cooperation was needed to avoid a world crisis. The commission, chaired by West German former chancellor Willy Brandt, also urged new North-South talks on such subjects as trade, food, energy and finance. The Cancun summit grew out of that recommendation.