Ethiopia today opened a constituent congress for the establishment of a full-fledged Communist Party to run the country in place of its present ruling military council under Mengistu Haile Mariam.

Attended by 1,742 delegates from across the country, the five-day congress opened in a new $45 million, Finnish-built hall just across from the old palace of the last emperor, Haile Selassie, who was toppled by the military Sept. 12, 1974.

The congress opened with a seven-hour speech by Mengistu, who is chairman of the Provisional Military Administrative Council that has been ruling the country since the start of the revolution here. He is expected to become head of the new Marxist-Leninist party, which is to be called the Workers' Party of Ethiopia.

In his all-day speech, Mengistu reviewed the stormy course of the 10-year-old revolution, which has survived repeated political upheavals, civil war, a secessionist struggle in northern Eritrea and Tigre provinces, an invasion from Somalia, persistent drought and constant economic difficulties.

He made it clear that Ethiopia intends to tighten its already close alliance with the Eastern Bloc, follow the communist model of economic and political development and join in its opposition to U.S. policies around the world. He accused the United States of trying to encircle Ethiopia by doubling the size of its Rapid Deployment Force and setting up a nuclear base on Diego Garcia island in the Indian Ocean.

The only matter on which he suggested Ethiopia and the United States have similar views is the recent mysterious placement of mines in the Red Sea. Mengistu said Ethiopia views the mining "as a source of considerable concern" and "vehemently opposes and condemns this illegal act."

On internal issues, Mengistu denounced the Eritrean secessionist effort and made clear he would not give up the struggle to crush it, although six offensives there have failed.

Mengistu also mentioned several times the "catastrophic" drought that has afflicted Ethiopia off and on for 10 years.

Present for the opening session in the heavily guarded congress hall were five African presidents and one Arab president plus representatives of more than 50 Communist parties from eastern and western Europe, the Arab world, Latin America, the United States and Canada.

Leading the Soviet delegation is Grigory V. Romanov, a Politburo member. The Soviets, who provide almost all of Ethiopia's arms and have become its chief ally, were instrumental in helping the military here to organize a Marxist-Leninist party, a task that has taken the best part of 10 years.