China has objected to a plan proposed by the five-member Association of Southeast Asian Nations and supported by the United States that calls for an end to hostilities in Cambodia and establishment of an interim government as a prelude to free elections.

China's objection to the ASEAN draft declaration on the interim-government issued emerged as the chief sticking point in the third day of talks among the 83 nations attending a United Nations-sponsored conference to develop a strategy for inducing Vietnam to withdraw its troops from Cambodia.

The current Cambodian government of Heng Samrin was forcibly installed by Hanoi after Vietnam's military invasion of Cambodia in December 1978.

The ASEAN draft, which has become the main working paper at the five-day conference, calls for a U.N. peacekeeping force and emphasizes the need to disarm all conflicting forces in the country as a precondition for the holding of free elections.

China, which supports the Pol Pot government Hanoi overthrew, opposes disarming the 30,000-man guerrilla force loyal to Pol Pot that is fighting Vietnam's occupation army. In its own draft proposal presented as a counterweight to ASEAN's plan, China insists only on "the disarming of Heng Samrin's establishment of an interim government in Cambodia under U.N. supervision pending elections. It also suggests the possibility of international economic aid to Hanoi to help persuade Vietnam to go along with a peaceful resolution to conflict in the war-torn country.

Peking opposes the establishment of the interim government, which would presumably weaken the claim of Pol Pot's forces that they represent the legitimate governing organization in Cambodia. In its hard line against Hanoi, China also resists the offering of economic aid to Vietnam.

The ASEAN nations -- Singapore, Malaysia, the Philippines, Thailand and Indonesia -- are the driving force behind the conference. T.T.B. Koh, Singapore's U.N. ambassador, said the five-member bloc favors "free elections under U.N. [supervision] in which all Kampuchean parties should participate. No armed faction or party would be in a position to repudiate the results of the election."

China, on the other hand, insists that "all the patriotic forces in Kampuchea consult among themselves and work out the necessary measures to ensure truly free elections, free from armed coercion." The Hanoi-backed Heng Samrin government presumably would be excluded from this process.

Koh suggested that ASEAN and the more than 20 other nations that have had a hand in working out the ASEAN draft declaration would remain flexible in the wording of the final conference document. But he said, "the essence must be maintained." More then 30 countries have regularly taken part in the daily working group discussions designed to produce a final resolution, which is expected to be adopted before the U.N. General Assembly conference ends Friday.

The British foreign secretary, Lord Carrington, speaking on behalf of European Community nations today, may have suggested the form the final resolution might take to break the current impasse. Carrington said: "Any settlement must satisfy two requirements. First, it must give the people of Cambodia the chance to determine freely the form of government and leadership they want. Secondly, it must embody the necessary understanding as to Cambodia's future relations with her immediate neighbors, and also with more distant powers who have exercised or may seek to exercise influence in that country."

The originally stated aim of the conference, to try to find a "cambodian problem, has been hampered because Vietnam, the Soviet Union and 24 other countries, many aligned with Moscow or Hanoi, have refused to participate.

Ha Van Lau, Vietnam's ambassador to the United Nations, said in a press briefing today, "This conference is one-sided" and "is in fact interference in the internal affairs of . . . the People's Republic of Kampuchea." As a result, he said, any declaration of the conference will be considered illegal and invalid by his government.

One senior ASEAN diplomat said that although no comprehensive settlement would emerge from the conference, it should render a "clear resolution" that would be useful in maintaining the pressure of international opinion on Hanoi favoring the withdrawal of Vietnamese troops from Cambodia. In adopting a more conciliatory line than previously held toward Vietnam, senior ASEAN diplomats said they would continue to seek a dialogue with Vietnam once the conference was over.