A newly reconstituted Bolivian congress elected veteran populist Hernan Siles Suazo tonight to take over as president following the armed forces' abandonment of a collapsing national government.

Siles Suazo, a leftist who won a plurality in a 1980 election, thus helping to prompt the military takeover, received 113 of the 146 votes cast in the congress. Two years ago, fearing a victory by Siles Suazo, the military disbanded congress to prevent it from meeting to choose a president as is required when no candidate wins an absolute majority.

But two weeks ago, despairing military leaders called congress into session and pledged to transfer power to the new civilian government next Sunday.

Military leaders, long isolated internationally because of reported human rights violations and alleged involvement in Boliva's cocaine trade, decided to give up power after they were unable to control the government's collapsing finances or a series of strikes that had paralyzed the country by mid-September.

Today's election marked the fourth time in as many years that Bolivia's moderate left political leadership has attempted to win power democratically in this poor, landlocked country in the center of South America. The unequal struggle has already led to three general elections, nine presidents and three violent coups since 1978, backed by varying alliances of generals, neighboring military governments and cocaine traffickers.

Siles Suazo, who served as president between 1956 and 1960 during the single sustained democratic interval in Boliva's coup-riddled history, has pledged to avoid further conflicts with the military and lead a moderate, conciliatory government that would focus on rebuilding the shattered economy.

From his exile headquarters in Lima, Peru, where he said he will remain until later this week, the 69-year-old leader has remained vague on his specific plans for government while seeking to assuage uneasiness among the military and neighboring authoritarian governments over his electoral coalition, which includes the Bolivan Communist Party.

In one recent interview, Siles Suazo, whose own party has been characterized by nationalism and social democratic ideology, was quoted as saying that the most signficant aspect of his election was "the broadening of the democratic front in South America's southern cone, which has been so damaged by military coups."

Siles' vice president-elect, Jaime Paz Zamora, said here yesterday that the new government "does not accept socialism and our program will not contain anything that is foreign to our country." The leader of the Communist faction in the coalition also pledged this week that Boliva's government would not be "reddened."

One of the new administration's major goals will be to win the diplomatic support and aid long denied the military, and U.S. officials here have indicated that aid will be increased after the installation of the civilian government.

The belated takeover of Siles Suazo will follow one of the most violent and corrupt periods of Bolivan military rule. Gen. Luis Garcia Meza, who led the July 1980 coup, earned international opprobrium and a cutoff of most foreign aid because of a violent campaign against the military's political opponents and the well-documented involvement of high military officers in smuggling and narcotics trafficking.

Garcia Meza was ousted by a palace coup in August 1980 but his successors, Gens. Celso Torrelio Villa and current President Gen. Guido Vildoso Calderon, did not contain either the corruption or deterioration of the economy, despite the mild support and frequent coaching from the U.S. govennment.

After weeks of antigovernment strikes and demonstrations, the expected election of Siles Suazo produced mild celebratory demonstrations today.

Enthusiasm over the apparent governmental change has been dampened here, however, by a series of strikes, financial crises and violence that threaten to overwhelm the new govenment.

Although the military leaves humiliated by the accession of the president their coup sought to block, the generals have bequeathed to their old civilian adversaries $40 million in overdue international loan payments, a nearly empty treasury, and an ongoing strike for higher wages by about 50,000 miners.

In addition, the powerful network of narcotics traffickers--which has turned cocaine into Bolivia's biggest industry under military rule-has taken advantage of the turbulent situation to attempt an effective takeover of large parts of the country, according to diplomats and military officials here.