The drive by Ethiopia's military ruler to establish a communist state is "irreversible" and a massive purge of liberal elements is under way before establishment of a "people's democratic republic" there this September, according to the Marxist nation's highest-ranking defector.

Dawit Wolde Giorgis, former head of the Ethiopian famine relief agency, urged in an interview this week with The Washington Post that the United States take a "very clear, strong stand" against the government of Mengistu Haile Mariam, including imposition of economic sanctions.

Dawit he said he still favors continued U.S. humanitarian assistance if it is strictly monitored and does not support Mengistu's socialist policies.

The United States is providing 300,000 tons of food this year, one-third of the estimated amount needed to feed 5.8 million drought-affected peasants. However, the Reagan administration is debating whether to continue humanitarian aid beyond this year and begin providing military aid to anticommunist opposition groups.

"Ethiopia is heading for . . . economic and political integration with the Soviet Union. I think it's irreversible," said Dawit, formerly a close associate of Mengistu and governor of the war-torn northern province of Eritrea for three years.

He said the Reagan administration should not take seriously Mengistu's occasional statements suggesting interest in establishing better relations with the United States. Instead, he said, it should impose economic sanctions and support "democratic forces" in opposition.

Dawit, granted political asylum here, said "ultra radical" Marxist elements on the 11-man, Soviet-style Politburo ruling Ethiopia have consolidated control and are systematically purging liberals.

In the last three months, he said, "hundreds of people" have disappeared, including many middle-level officials. Among recent detainees in Addis Ababa is Berhanu Dinka, Ethiopian ambassador to the United Nations, according to reports here.

Dawit said several officials defected recently, among them Berhanu Deressa, his top aide in the Relief and Rehabilitation Commission. Deressa is in the United States.

Commercial attaches Abebe Worku, stationed in New York, and Hailu Tefara, of the Ethiopian Embassy in London, have defected, along with the ambassdor to South Yemen and two department heads in the foreign ministry, he said.

Dawit, who left Addis Ababa Oct. 25 on a tour of Europe and the United States and never returned, said he waited until early May to speak out publicly against Mengistu's government because he had been privately trying to resolve differences with Mengistu.

Dawit said he thinks that Mengistu has lost contact with Ethiopia's "economic and political realities" and is seeking to build a "personality cult" in the image of North Korean ruler Kim Il Sung, whom he said Mengistu greatly admires.

Dawit broke his silence after Ethiopia accused him publicly of embezzling more than $300,000 donated by U.S. private relief groups and deposited in bank accounts in the United States.

He said the charges are "totally false" and that the contributions in dollars had been used to buy relief supplies here, while donors were credited with an equivalent amount in Ethiopian currency in their accounts in Addis Ababa.

He said Ethiopian embassies in the United States and Europe purchased relief supplies and paid government bills in this way. "I have the documents to show how the money was spent," he said.

Dawit said he disagreed with the way the Mengistu government tried to resettle hundreds of thousands of famine-afflicted peasants from northern Ethiopia in new southern villages and with its policy of collectivization, now affecting 3 million peasants.