United Nations, N.Y. — The assassination of Indian Prime Minister Indira Gandhi dealt a severe blow to the movement of ''nonaligned'' countries, which she led. This group of 100 nations - which has been torn apart by the polarizing tendencies of the superpowers - is deprived of strong, experienced, and credible leadership, senior third-world officials say.
When Mrs. Gandhi took over the nonaligned leadership from Cuban leader Fidel Castro last year, many moderate nonaligned leaders felt relieved. Cuba had frequently tried to pull the movement closer to the Soviet Union, claiming that a natural alliance existed between the nonaligned and the communist bloc.
Pure nonalignment is of course impossible, because every country must work within its own economic, climatic, and geographic circumstances. India tilts toward the Soviets regarding Afghanistan, toward Vietnam regarding Kampuchea.
''But fundamentally India tried to restore the nonaligned group of nations' middle-of-the-road position of refraining from supporting either superpower,'' says an observer of the nonaligned movement.
Rajiv Gandhi, the new Indian prime minister, will complete his mother's four-year term as nonaligned leader. This concerns some diplomats.
''Rajiv Gandhi has other fish to fry than to steer the nonaligned movement. And furthermore he has little or no experience in this area. So we are all worried,'' says a senior nonaligned diplomat.
The nonaligned movement has been weakened for various reasons:
* It has been unable to stop the war between two nonaligned nations, Iran and Iraq.
* In the context of the new Soviet-American cold war, many nonaligned nations felt the need to move closer to one of the superpowers for military and economic protection.
* Its legitimacy has come under attack. The Reagan administration refuses to acknowledge the group's existence, calling it ''the so-called nonaligned.'' The Soviet Union considers the third world to be essentially divided between ''liberated'' countries (like Angola, Cuba, and Ethiopia) and ''annexes of the capitalist world'' (all the others).
Mrs. Gandhi tried hard to aim nonaligned nations toward achievable ends rather than toward pipe dreams, nonaligned diplomats here agree.
She pleaded in favor of a ''world conference on monetary and financial matters'' rather than for a ''global negotiation'' on economic affairs, which the West did not support.
She brought a number of Western, Eastern, and third-world leaders together last year at the UN for informal talks on how to bridge some of the gaps on North-South and East-West issues.
She meant to strengthen the role of the UN and of multilateral diplomacy.