By LARRY ROHTER
 
LA PAZ, Bolivia, Oct. 17 — Gonzalo Sánchez de Lozada resigned as president of South America's poorest and most unstable country late Friday after nearly a month of increasingly violent clashes between army troops and predominantly Indian demonstrators that have left more than 80 people dead.

A special session of Congress formally approved the step in a vote just after 10 p.m., and there were conflicting reports on Friday night as to the former president's whereabouts. Some television and radio stations said he had flown by helicopter to Peru, while others reported that he was boarding a commercial flight to the United States from the eastern city of Santa Cruz.

Mr. Sánchez de Lozada, 73, an advocate of free-market policies, had been under increasing attack here because of a proposal to export natural gas to the United States through a port in Chile, Bolivia's traditional enemy.

Last weekend he ordered troops to suppress the unrest, a move that has led to mounting deaths and more resistance to his rule. On Friday the last pillars of his political support crumbled, and Mr. Sánchez de Lozada reluctantly took the step that he had always insisted he could never take because it would fatally wound democracy here.

With his resignation, Mr. Sánchez de Lozada became the fourth South American president to be forced out of office by popular unrest since 2000. Jamil Mahuad of Ecuador, Alberto Fujimori of Peru and Fernando de la Rúa of Argentina were the others, and in all but Mr. Fujimori's case, unpopular economic policies were also the main factor in their fall.

In the letter offering his resignation, Mr. Sánchez de Lozada attributed his undoing to what he called a "seditious" movement seeking to overturn the verdict that voters had delivered when they elected him last year, and he also warned that the crisis was not over. "The dangers that hang over our fatherland continue intact," he wrote.

That political passions are still running high became clear during the joint session, which started more than five hours late because many legislators were delayed by roadblocks and blockades, and needed a military escort. During a minute of silence for victims of the violence, some opposition legislators cried out "murderer, murderer," and the first attempt to read Mr. Sánchez de Lozada's letter out loud failed because it was drowned out by shouts, whistles and personal insults.

Nevertheless, Hormando Vaca Díaz, who as president of the Senate presided over the joint session, described the resignation as "a solution that is not ideal, but which preserves the constitutional order, which had been at risk." Other legislators and political analysts expressed relief that this nation of eight million had managed to avert a civil war or a military coup.

Even before legislators voted 97-30 to accept the resignation letter, jubilant protesters at the Plaza of Heroes, one of the main sites of recent demonstrations, cheered, waved flags and burst into song and dance. "Goni is gone, Goni is gone," they chanted, referring to Mr. Sánchez de Lozada by his nickname.

As stipulated by the Bolivian Constitution, Mr. Sánchez de Lozada was replaced by his vice president, Carlos Mesa. A 50-year-old historian and television journalist who gained national popularity as the host of a program called "Up Close," Mr. Mesa was initially chosen as Mr. Sánchez de Lozada's running mate because he belonged to no political party and had a reputation as a corruption fighter.

Once he took office, however, Mr. Mesa clashed with many of the career politicians with whom he will have to deal as president. He broke with the government earlier this week, but with no political base of his own, he has sought to position himself between the warring camps and to offer himself to the public as a neutral healer and conciliator.

"I am not with the philosophy that reasons of state justify death," he said at his last public appearance, on Thursday. "But neither am I with the radical banners that the moment has arrived to destroy everything in order to construct a utopia that nobody wants or knows where it is going."

From the city of Cochabamba, Evo Morales, leader of the coca growers' federation and runner-up to Mr. Sánchez de Lozada in the election last year, said he would support the "constitutional" solution that would make Mr. Mesa president. "Let's hope that Carlos Mesa understands all of the demands that have been put forward by the people," Mr. Morales said Friday night after the vote, in which he chose not to take part.

The position of the other principal Indian leader, Felipe Quispe, whose organization began the demonstrations, was less clear. He refused to pledge support to Mr. Mesa when reporters asked him on Friday afternoon what the bloc he controls in Congress would do, though his followers eventually voted in favor of the change of government.

Mr. Sánchez de Lozada's last day in office began with the news that his spokesman, Mauricio Antezana, had resigned. Shortly afterward, the leader of one of the parties in the precarious governing coalition, Manfred Reyes Villa, a former army captain, emerged from a meeting with the president and announced that his group, the conservative New Republican Force, was also withdrawing.

"I'm not one to jump off the boat, but we can't go on rowing against the tide, against an entire people who are seeking his resignation," Mr. Reyes Villa said.

"We are not going to be part of the biggest massacre of the people," he added. "We can't go on like this."

On the streets, the demonstrations, which have mostly been led by indigenous movements, appeared during the day to be strengthening, with even middle-class neighborhoods joining in. Elegantly dressed women banged pots and pans in protest, and as a sign of mourning for those killed over the past week, many businesses hung the red-yellow-and-green Bolivian national flag with black ribbons added in the center or black bunting put around the edges.

"Goni has got to go," said Mirta Camacho Valdivieso, a middle-class housewife alarmed by the food shortages and general chaos. "Too much blood has been shed and too many hatreds awakened for him to continue in office."