The surprise selection of Col. Yhombi Opango as new president of the People's Republic of the Congo last weekend could lead to major changes in the traditionally Marxist government, offering new hope for internal stability and reestablishment of relations with the West.

Opango succeeds the late Marien Ngouabi, who was assassinated on March 18, allegedly by four men hired by former President Alphonse Massemba-Debat.

Col. Opango, 38, is the first moderate chief of state since the first Congo president, Abbe Fulbert Youlou, was overthrown in 1963.

Diplomatic sources and Congowatchers believe Opango's appointment could lead to a thaw in the previously cool relations with the United States and other Western countries. He is said to have strong ties in military circles in France, where he was educated, and a strong interest in balancing current. Eastern bloc ties with links to the West.

The selection, announced Sunday by the military committee may also avoid an anticipated flare-up of fierce tribal rivalries.

(The Congo abolished the main principles of the 1973 constitution Tuesday concentrating full powers in the military committee, according to a Radio Brazzaville announcement monitored in Yaounde, Cameroon, by teh Associated Press. A key move was to give the military committee power to name the president.)

Opango was born in Owando in the same area as Ngouabi among the northern M'bochi tribe, but his mother - through whom tribal ties are passed - was a Lari from the southern region around Brazzavelle, the capital. Thus the new chief of state is considered a kinsman' of the slain former president while offically classified as a member of the rival tribe.

The Laris have not held power for 14 years. Since the Youlou administration was overthrown, the M'bochi tribe, more militant and leftist, has dominated the government. The majority of the military committe is M'bochi and the two frontrunners for the presidency - Maj. Denis Sasous Nguesso and Maj. Sylvain Ngoma - are notherners.

In the new government line-up, Ngoma has been named premier and second vice president, and Nguesso becomes first vice president and minister of defense and internal security.

Before the appointments were announced, there was fear that rising animosities in the former French colony would explode during the selection of a new leader. Ngouabi's assassination triggered a wave of tribal killings across the country.

Emile Cardinal Biayenda, Roman Catholic archbishop of Brazzaville, was among the victims. Massemba-Debat was also executed shortly after allegedly confessing his part in the assassination plot.

It appears that Opango was a dark-house compromise, a man picked because he might be able to unite the nation. He rose to national fame after putting down a coup attempt in the early 1970s, but recently he had apparently been out of favor.In December 1975 he was moved from the top army post to the more obscure Ministry of Public Works.

His family political history may have helped maintain an image. Opango's father was a candidate for the presidency in the first election in 1960 when the Congo - a country about the size of New Mexico - gained independence.

His academic credentials are also impressive in a country where 80 per cent of the 1.3 million polpulation is illiterate. The new president was educated at the French military preparatory school at Strasbourg and at the Saint Cyr, France's West Point.

Before becoming army chief of staff, he served as military attache in Moscow.

Besides the antagonisms among the country's rival tribes and political factions, Opango will inherit many other problems from Ngouabi: high unemployment, economic underdevelopment, low living standards, and lack of capital and trained personnel.