LA PAZ, Bolivia -- Protesters threw up road blockades of giant boulders and burning mounds of trash in and around Bolivia's capital Tuesday, demanding the president resign after days of deadly rioting and clashes.

With stocks of food, gasoline and other basics dwindling, La Paz ground to a standstill. Leaders of labor unions and Indian groups called for more protests, a day after clashes between demonstrators and soldiers killed at least 16 people.

Demonstrators, including Indian women in black bowler hats, clutched rocks and stood defiantly on bridges spanning the main highway connecting the capital, La Paz, with the nearby city of El Alto, menacing the few motorists who dared to drive the streets.

Hundreds of others, including men and children, stood behind a highway fence shouting "What do we want, companeros?" The group shouted back: "The president must go!"

President Gonzalo Sanchez de Lozada, who has refused calls to step down, remained secluded in his heavily guarded residence. The crisis began about three weeks ago when his free-market plan to export natural gas to the United States and Mexico ignited long-simmering discontent with his democratically elected 14-month-old government.

Bolivia has 8.8 million people, but its poor Indian majority has been showing growing disenchantment with free market reforms and U.S.-backed plans to eradicate coca leaf growing in this Andean mountain nation.

Sanchez de Lozada said Monday he was freezing the gas project, but loudspeakers reverberated with calls Tuesday for people to take to the streets again after protests in La Paz, El Alto, Cochabamba and other cities in recent days.

One demonstrator, who gave his name only as Valentin, called the gas export plan "a project concocted by a government that wants to line its pockets." He appeared unconvinced the project had been frozen.

"Why should we sell the gas? We Bolivians could use it to heat our homes," said the angry man, wearing a New York Yankees cap.

"Bolivia will only prosper once 'Goni' is gone," shouted one man over a loudspeaker, referring to the president by his nickname.

Lady Gutierrez, a 20-year-old student, said she was planning to join the demonstrations because she felt the president wasn't helping the poor.

"There's too much poverty in Bolivia and too few people who want to do something to about it," Gutierrez said. "We're angry and hungry for a change."

As many as 60 people have been killed in three weeks of protests, according to human rights groups and local media. Authorities have not confirmed this figure.

Many protesters spent Tuesday building or strengthening barricades that have virtually strangled La Paz and El Alto. Shops, banks and offices were closed in the two cities, an urban sprawl on the high Bolivian altiplano that is home to about 1.5 million people. The few people going to work had to walk.

At the El Alto International Airport, anxious-looking foreigners boarded charter flights out of the capital. Routine commercial flights were halted over the weekend amid security concerns for travelers arriving here.

The president made no public appearances Tuesday, ensconced in his home instead of going to the presidential palace in the downtown Murillo plaza, which also was under heavy military guard. Four small tanks were deployed in the plaza and soldiers with automatic weapons stood by.

Sanchez de Lozada, 72, says demands for his resignation are part of an international plot to topple his government.

A U.S.-educated millionaire, Sanchez de Lozada served as president from 1993 to 1997 and took office for a second term in August 2002 after narrowly defeating Evo Morales, a radical congressman and Indian leader of Bolivia's coca growers.

A wealthy mining executive who has the backing of the United States, he implemented an aggressive capitalization program that partially privatized many of Bolivia's state-owned industries in his first term. For his second term, he promised a similar free market plan to rebuild South America's poorest nation.

Sanchez de Lozada won last year's election only 22 percent of the vote.

His popularity appears to be high among the military, but it has faltered elsewhere. His vice president, Carlos Mesa, withdrew his backing Monday, citing tactics used to quell the protests.

Mesa did not resign, although Economic Development Minister Jorge Torres quit his post, also protesting government moves to repress the street protests.

On Monday, the president went on television and vowed "to defeat the sedition and restore order." He called the massive protests "a plot encouraged from abroad aimed at destroying Bolivia and staining our democracy with blood."

Although he did not elaborate, Sanchez de Lozada blamed his former opponent Morales and another indigenous congressman -- Felipe Quispe -- for the alleged plot to oust him. Both denied the accusations but insisted that he must go. The president's five-year term ends in 2007.

Morales said Sanchez de Lozada should be succeeded by "an indigenous government."

Coca is the base ingredient of cocaine, but many Bolivians chew the leaves or use them to brew tea. About 30,000 acres of coca can be cultivated legally, but growers want the limit increased.