While Fidel Castro spent today welcoming heads of state arriving for Monday's nonaligned summit conference here, diplomats of the 93-member organization were locked in an increasingly bitter dispute about what some perceived as an attempt by their host to alter the organization's membership list.

At issue is which of two delegations from Cambodia, where Vietnamese-supported rebels last winter drove out a government still officially recognized by the nonaligned organization, should occupy Cambodia's seat here.

An ally of both Vietnam and the Soviet Union, Cuba joins them among the relatively few nations that officially recognize the new People's Republic of Cambodia government. Cuba granted visas to delegations from both the new leadership and the old Democratic Cambodia government, which is still represented at the United Nations. However, when representatives of overthrown president Pol Pot's government tried to attend a summit preparatory meeting last Tuesday, Cuban officials turned them away.

Since then, in organizing meetings and the three-day foreign ministers' conference ending today, the nonaligned countries have talked of little else. In a speech to a closed ministers' session yesterday, Yugoslav Foreign Minister Josip Vrhovec said Cuba's unilateral action questioned "the very essence of the policy of nonalignment. The consequences would be highly dangerous for many member states."

Singapore's Foreign Minister Sinnathamby Rajaratnam was more explicit.

"The question of who should or should not be invited to meetings of the nonaligned movement should rest with the foreign ministers or with the heads of state," he said. "It should not be left to the whims and fancies of host countries."

For many member states, the Cambodian issue is a metaphor for a deep division at the core of the nonaligned movement.

Among the movement's founding principles is a condemnation of big power imperialism and avoidance of East-West battles and military alliances. Many nonaligned members feel that both the United States and the Soviet Union should be avoided.

Cuba, however, leads a faction pressing for recognition of the socialist world -- which the others interpret as meaning the Soviet Bloc, although Cuba avoids saying so -- as "natural allies" in the nonaligned struggle against Western domination.

At past summits, Cuba has failed in attempts to make this distinction a part of nonaligned policy. An opposing group, led by the movement's founder, Yugoslavia, has criticized Cuba for its close Soviet ties.

Now, as host of the sixth nonaligned summit and author of the principal summit declaration, Cuba has put its beliefs into practice.

When Cambodia's seat turned up conspicuously empty at the opening day of preliminary meetings last week, and the Pol Pot delegation issued a declaration saying the Cubans had kept them out, the conference erupted.

While Pol Pot's government had earned universal opprobrium for its widely reported genocidal policies, many nations were disturbed by what they saw as blatant Vietnamese intervention to install a pro-Soviet government last February. Cuba, on the other hand, views the Cambodian situation as precisely the sort of liberation struggle the movement supports -- an example of how the Soviets are friends rather than foes of the nonaligned movement.

The United Nations, having trouble deciding the question, continues so far to recognize the Pol Pot government. In summit preparatory meetings over the past year, the nonaligned countries were unable to agree on what to do. Instead, according to the faction led by Yugoslavia, they agreed temporarily to accept the status quo, allow a Pol Pot representative to be seated at their discussions without a voice, and deferred action until the summit.

Last Tuesday and Wednesday, the preparatory committee quickly became entangled in what the Yugoslav and Singapore faction feels is a kind of diplomatic double talk by which, as they see it, the Cuban faction hopes to get its way.

As the preparatory committee's sessions began, Malaysia pointed out that the Democratic Cambodia delegation "had been prevented by the Cuban authorities from taking its rightful place at the present meeting," reminded delegates of the earlier decision to accept the status quo and "urged the delegation be seated forthwith."

Cuba's ally, Laos, disagreed.

Gabon, Burma, Morocco, Zaire, Yugoslavia, Nigeria and Bhutan seconded Malaysia. At preparatory meetings in Sri Lanka and New York over the summer, they said, the movement had agreed to accept the status quo until the summit.

Cuba disagreed. The only thing the previous meetings decided, its representative said, was that the organization could come to no decision. Therefore, "the only real status quo was the absence of consensus" on what to do. His government, the Cuban representative said, had decided to issue visas to both delegations, and either of them could be seated as soon as a final decision had been reached.

Mali, Togo, Sudan and the Comoros objected. Somalia said that Democratic Cambodia's absence was a "violation" of nonaligned principles, with which Cuba had "unilaterally interferred."

Ethiopia said no previous consensus on what to do had been reached, so Cuba was right in doing nothing by seating neither delegation. Vietnam agreed.

Singapore disagreed and said that excluding the Pol Pot delegation was, in effect, making a decision.

Cuba noted that it had complied with its commitment to be impartial until the foreign ministers or heads of state made a decision "in spite of the repugnance that the crimes committed by the Pol Pot clique provoked" in its government and people. The arguments continued tonight in hotel rooms, restaurants and hallways among groups of delegates.

"The nonaligned movement is going down the drain," one delegate complained. Despite nearly everyone's dislike of the Pol Pot government, he said, the whole thing would fall apart if such impromptu actions (as Cuba's) were permitted.

"They call us wishy-washy liberals who want to follow procedures, while they get rid of Democratic Cambodia and every other country the Soviets want," this delegate said.

But a Cuban source contended that by sticking to outmoded rules and failing to recognize that the new government controls Cambodia, countries such as Yugoslavia were "taking the side of a fascist regime."

Some delegates feel that Cuba is using the Cambodian issue to monopolize the talks, keeping delegates from having sufficient time to debate and possibly reword the controversial summit declaration drafted by Cuba. The Cubans deny any such intent.