The ''accuse and demand'' tactics of many of the world's poorer nations, which have built up a sense of confrontation between the ''have'' and ''have not'' countries in the last two decades, are giving way to a more conciliatory mood.
Stridency and wild-eyed radicalism are out. Pragmatism is in. And India, which took over the rotating leadership of the nonaligned group from its more extreme predecessor, Cuba, is consciously pushing a more moderate stance.
''Constantly baiting the West when the West is also in economic difficulties and when their help is needed to assist the third world is seen to be counterproductive,'' says a Western diplomat at the UN.
This new spirit of cooperation took many Western diplomats by surprise at the last UN General Assembly in the fall. But it has persisted through meetings of the nonaligned bloc at New Delhi last month and of the Group of 77, the economic forum of developing countries, at Buenos Aires this month.
All this is fueling optimism here that a more accommodating spirit will prevail at the next session of the United Nations Conference on Trade and Development (UNCTAD) when it meets June 30 in Belgrade, Yugoslavia.
UNCTAD VI, so called because it will be the sixth such quadrennial meeting, will be the world's largest conference on trade. It will bring together more than 3,000 participants from well over 100 countries.
The less hostile tone at recent nonaligned forums is cause for some relief for the Western powers, which have been the target of the developing world's frustration. At the same time the United States, which under the Reagan adminstration has bridled at the nonaligned's persistent attacks on Washington, remains deeply skeptical about a change of heart in the third world.
To one US delegate who attended the New Delhi nonaligned meeting, the seemingly more accommodating approach taken by third-world countries on economic matters is negated by the continuing harsh political attacks on the US. And to this delegate both political and economic issues have to be seen as ''part of one package.'' Thus the US diplomat feels it will take much more than a more cooperative spirit at Belgrade to erase years of mistrust and suspicion.
Previous UNCTAD meetings have stumbled on the rocks of poor-nation rhetoric, with the developing countries accusing the developed countries of perpetuating a global economic system that protects the interests of the industrialized world at the expense of the developing countries. Demands for a larger share of world trade and an easing of rich-nation protectionism through the removal of prohibitive tariffs and quotas have fallen largely on deaf ears.
But Gamani Corea, the Geneva-based Sri Lankan secretary-general of UNCTAD, feels the world recession, hitting both developing and developed countries, will bring a sense of interdependence at the UNCTAD VI meeting that has been absent from past sessions.
To Mr. Corea, who has held office since 1973, UNCTAD VI marks the first time since the conference evolved in 1964 in which all the groups will attempt to find a way out of the impasse. ''For the first time we have sufficient basis for common interest,'' he told journalists at UN headquarters.
''I have hopes that Belgrade will take on a more hopeful character than its predecessors and that we will not indulge in sterile debate.''
One indication the nonaligned countries are intent on a less confrontational course is a growing tendency within the movement to try to adapt and refine rather than restructure the world economic system and its major institutions.
And there is a readiness this time to accept a more global approach by adopting a broad-ranging agenda that underscores the interdependence of world economies. This is in sharp contrast to previous UNCTADs, which have tended to concentrate on one major topic - generally the exclusive interest of the ''have not'' countries. UNCTAD II (New Delhi, 1968), for instance, called for a general system of trade preferences for developing countries. UNCTAD IV (Nairobi, Kenya, 1976) made a plea for a common fund for commodities that would provide a common pool of money to help prop up world prices of commodities. Developing countries were among those hardest hit by sagging commodity prices.
Mr. Corea takes the view that it is precisely because the world economy is in such difficulties - because world trade has stagnated and diminished in the past two years - that this UNCTAD will find a more cohesive spirit at the conference.
The we're-all-in-this-together syndrome for both developed and developing countries has also been echoed by World Bank president A. W. Clausen. Mr. Clausen urges an open trading system in the world since, he says, if the developing nations cannot sell, they cannot buy, and ''thus everyone is a loser.''
At the meeting of the Group of 77 in Buenos Aires, papers were presented showing that after 40 years of expansion, world trade had dropped 6 percent in 1982. Prices for raw materials exported by developing countries are in real terms at their lowest in 50 years. At the end of 1982, third-world debts amounted to $630 billion.
UN observers say the overall decline in the world economy has made the developed world more responsive to the problems of the third world, especially now that Western banks have been forced to bail out key third-world nations like Mexico.
As Mr. Corea put it: ''One can't think of a healthy world economic situation if stagnation is taking place in three continents.''