Three-time president Victor Paz Estenssoro emerged tonight as the virtually assured winner in Bolivia's dragged-out election during a raucous congressional session that his opponents denounced as "a farce" marred by fraud.

Congress debated late into the night before taking its required runoff vote, but Paz's victory seemed assured earlier when he assembled a center-left coalition representing a majority of votes in the 157-member Congress.

The vote was delayed when legislators backing ex-president Hugo Banzer, filibustered late into the night. Banzer's backers, foreseeing defeat, earlier walked out of the joint session of Congress and announced that they would refuse to vote. They returned after a recess and started their delaying tactics.

The silver-haired, bespectacled Paz, 77, trailed Banzer in the popular vote of July 14. Rightist ex-general Banzer, who seized power in a 1971 coup and ruled until 1978, had led 2 to 1 in initial returns. But as the vote trickled in from strongholds of Paz's National Revolutionary Movement in the countryside, Banzer's lead dwindled to just over 2 percent.

By law, the Congress chooses among the leading candidates if none wins an absolute majority in the popular vote. Banzer, 59, who won 28.6 percent of the 1.7 million votes tallied, and Paz, with 26.4 percent, had to go hat in hand to the myriad smaller parties to try to patch together a coalition for the vote in Congress. Paz's party had won 59 of the 157 congressional seats, against 51 for Banzer's.

The new president is scheduled to be sworn in Tuesday. Five Latin American presidents are to attend, showing support for the democratic transition in this country of 6 million inhabitants that is the region's record holder for coups. Assistant Secretary of State Elliott Abrams is to represent the United States.

Paz's victory could result in political turmoil at a time when the country, with the lowest per capita income in South America, is in severe economic crisis. His allies admit that he will need strong support to rein in inflation and stimulate economic growth after nearly five years of severe recession.

Paz's last term as president ended in a military coup in November 1964, just three months after the election. He was the principal leader of the 1952 nationalist revolution that resulted in nationalization of the country's crucial tin mines and a sweeping agrarian reform that benefited the Indian majority. The leftist revolution was considered unique at the time because its victors received U.S. support.

Now considered a centrist by supporters or an autocrat by opponents, Paz supported Banzer's coup in 1971 but was himself exiled two years later by the general's regime.

When Paz returned in 1978, he lost his next three bids for the presidency in the turbulent political years between 1978 and 1982, during which nine governments rose and fell. He is considered an able politican who keeps his own council.

Paz is part of a troika, along with outgoing President Hernan Siles Zuazo and flamboyant labor leader Juan Lechin, that has dominated Bolivian politics for most of the past four decades. He has pledged that this will be his last political office.

The session at the elegant Congress building was interrupted frequently as the candidates' supporters in the packed gallery traded jeers and shouted victory slogans.

Paz's bid got a boost early in the day when the influential leftist party of the third-place candidate, Jaime Paz Zamora, pledged its 16 legislators. Later, when legislators chose new congressional leaders and excluded Banzer's politicians, his Nationalist Democratic Action party led a walkout.

"We are the first majority in the country," said Ronald Maclean, candidate for mayor of La Paz on Banzer's ticket. "The people won and should rule. This is a farce and a violence against democracy.".

Supporters of Banzer charged that nearly 400 ballot boxes from La Paz, where he won heavily, had "disappeared." However, the electoral court rejected the claims.