By LARRY ROHTER
 
LA PAZ, Bolivia, Oct. 13 — Thousands of demonstrators marched in Bolivia's capital and other nearby cities on Monday, calling for the president's resignation. But they were dispersed by military units firing tear gas canisters, and at a midafternoon news conference President Gonzalo Sánchez de Lozada was defiant and pugnacious.

"I'm not going anywhere," he said, citing his determination to remain in office and vowing that "order will be restored." He added, "It is not possible that democracy be replaced by a dictatorship of the unions" that will "pit region against region, class against class and ethnic group against ethnic group."

Clashes between demonstrators and the military have killed at least 42 people. At least 14 of them were killed Monday, according to Bolivia's Permanent Human Rights Assembly.

With popular revulsion growing, leaders of two parties that have been part of the precarious governing coalition said Monday that they were thinking of pulling out. But the clearest indication of weakening support came when Vice President Carlos Mesa announced that he was breaking with the government, which had the support of only 8 percent of those asked in recent polls.

"Neither as a citizen nor a man of principles can I accept that, faced with popular pressure, the response should be death," Mr. Mesa said, although he said protest groups bore part of the blame.

Bolivia, the poorest country in South America, has been racked since mid-September by antigovernment protests initially organized by groups representing the Indian peasants who are the country's impoverished and marginalized majority. But labor unions, student and neighborhood groups and opposition political parties have since joined and helped strengthen the movement.

In recent days, with many people angry over a government-backed proposal to export natural gas that opponents say would not benefit most Bolivians, the protests have grown increasingly confrontational, with demonstrators armed with sticks of dynamite blocking highways. Over the weekend, Mr. Sánchez de Lozada called out the troops, an action that has raised the level of violence even further.

The immediate cause of the unrest is the proposal to build a $5 billion pipeline to begin exporting Bolivia's vast reserves of natural gas to the United States and Mexico through a port in Chile. Opponents worry about corruption and complain that the royalty rate on gas shipments is so low that the project will end up offering more financial benefits to foreigners than to this Andean nation of eight million people.

"We've always exported our natural resources, like silver and tin, to others, so that they get rich and we remain poor," said Luis Alberto Javier, 30, a plumber's helper who supports the protests. "That gas should remain here to create jobs in Bolivia for Bolivians rather than be sold abroad, especially through Chile."

Chile is viewed as an enemy here because Bolivia has been landlocked ever since it lost its outlet to the Pacific Ocean in a war with Chile in 1879. Rapid economic growth in Chile during the past two decades has increased Bolivians' resentment. Mr. Sánchez de Lozada, a 73-year-old millionaire businessman, is seen as being overly cozy with Chilean business interests.

In a television address to the nation early on Monday, he promised that no new gas exports would be permitted until the citizenry was consulted, and he called for negotiations "to try to reach a consensus." Opposition leaders, sensing that his position was rapidly deteriorating, quickly rejected his call.

"We are not going to have dialogue with the murderers of the people," said Evo Morales, who leads the powerful coca growers union and who finished a close second in the presidential election last year. After the "massacre" over the weekend, he added, the opposition's attitude toward the president is one of "resignation or nothing."

Police officers in some outlying areas of the capital have joined demonstrators, according to local news reports. The loyalty of the police has been in doubt since a nationwide mutiny in February.

Mr. Sánchez de Lozada apparently decided to take the police off the streets in working class suburbs like El Alto and replace them with army troops backed by tanks and helicopters for precisely that reason. But even the prospect of continued military support for his government was being questioned.

"The Armed Forces are reaching the limit of their tolerance for a situation in which they are being blamed for these deaths," said Juan Ramón Quintana, a former military officer who now leads a private institute called the Bolivian Program for Strategic Research.

With roads in and out of the capital blocked, gasoline scarce and renewed violence a threat, many residents of La Paz stayed home from work on Monday. Most flights from the main airport, in the area that has experienced the most violence, have been canceled or postponed, airport officials said, and many stores are running out of supplies.

At a small butcher shop here, the proprietor, Estela Mamán, said on Sunday that she was about to run out of meat, and would soon be forced to close down. Nevertheless, she said she supported the protests.

"The government is going to have to give in if there is to be a solution," she said. "None of this would be happening if they just listened to us, the people, but they never do, and now they are paying the price."