By JUAN FORERO

LA PAZ, Bolivia, June 8 - Bolivia braced for more tumult this week as Congress prepared to meet to accept President Carlos Mesa's resignation and anoint his successor, Senate President Homrando Vaca Diez, whom protest leaders vehemently reject.

Mr. Vaca Diez said he had convened a special session of Congress in the historic city of Sucre on Thursday at 10:30 a.m. because La Paz remained besieged by demonstrators. Lawmakers are expected to accept Mr. Mesa's resignation offer, which the president made in an emotional late-night televised speech on Monday to defuse mounting protests against the state's economic policies.

With Mr. Mesa's departure, Mr. Vaca Diez, a conservative from wealthy eastern Santa Cruz province, would become president. Protest leaders said that demonstrations would try to stop Mr. Vaca Diez or the man who is next in line to the presidency, Mario Cossio, the president of the lower house of Congress, from ruling.

"We will not allow them to take power," said Evo Morales, an influential congressman who heads a leftist party pushing for more indigenous rights. "Now is when the national majority has to govern the country."

Mr. Morales, an Aymara Indian who is leader of the Movement Toward Socialism party, said that if Mr. Vaca Diez becomes president "the government will be in the style of a dictatorship."

The impending confrontation between the left and the right in Bolivia foreshadows more turmoil in this Andean country of 9 million, where a restless and poor Indian majority has been protesting for months for more political power and nationalization of the private energy companies operating here.

Anger at the political and economic system in Bolivia is palpable, with many people seething because the country is no better off after implementing market reforms pushed by the United States.

That impatience with the so-called Washington Consensus has been evident across Latin America, where eight presidents have been ousted or forced to resign in popular uprisings since 2000. Left-leaning candidates have been swept into power in two-thirds of South America's countries.

"The bottom line is that Latin America is in open rebellion of the economic policies of the Washington Consensus," said Jim Shultz, director of the Democracy Center, a group in the Bolivian city of Cochabamba that is critical of free market reforms in the country. "Sometimes it happens in the ballot box. Sometimes it happens on the street, like in Bolivia. It is in essence the same rebellion."

Mr. Vaca Diez, in a press conference televised early today, signaled that the military must be used to restore order and he warned that the protests could lead to a right-wing backlash in the country, declarations that were interpreted by leftist protest leaders as warnings.

"The radicalism of the left leads to totalitarian governments," he said. Mr. Vaca Diez said if radical groups "push for confrontation and a blood bath this will always end in authoritarianism."

He did not explicitly say that he would remain as president when Mr. Mesa's resignation is accepted. But two powerful institutional parties, the Nationalist Revolutionary Movement and the Movement of the Revolutionary Left, have both thrown their support behind the Senate president.

Mr. Mesa, in a televised speech Tuesday night, pleaded with Mr. Vaca Diez to take himself out of the running for president.

Mr. Mesa and many Bolivians, including church leaders who this past tried to mediate a solution to the growing crisis, also want Mr. Cossio to pull out.

That would permit the third in line, Eduardo Rodriguez, the president of the Supreme Court, to take power. Mr. Rodriguez could serve as caretaker until early elections are convened. In a poll published today in La Prensa, a newspaper in La Paz, 55 percent of Bolivians said they would support Mr. Rodriguez, while only 16 percent said they would support Mr. Vaca Diez.

"The country cannot play with the possibility of breaking into a thousand pieces," Mr. Mesa said. "This is a plea, before a country that is on the edge of civil war."

Speaking directly to Mr. Vaca Diez, Mr. Mesa called on him to "resign his privilege to the constitutional succession."

"It is in your hands to show the country a generosity that will make history, that will make you closer to the people than to continue insisting on the impossible," Mr. Mesa said.

In the sprawling indigenous city of El Alto, high above La Paz, protests again gathered force this morning, with marchers preparing to descend into the capital in a show of force. On Monday and Tuesday, tens of thousands of Indians and miners flooded La Paz, clashing with police who repelled them with tear gas and water cannons.

"We are a step away from civil war," said the commander of Bolivia's national police, David Aramayo.