La Paz, Bolivia - Embattled President Gonzalo Sanchez de Lozada resigned Friday, a presidential aide said, hours after losing the support of his last key ally following weeks of deadly street protests over an unpopular plan to export natural gas.

Under Bolivia's constitution, Vice President Carlos Mesa would replace him, but is doubtful whether Mesa will be accepted by the masses in revolt. A former television reporter, Mesa is a political independent and a respected historian, but has no political base of his own.Earlier, as word of the president's impending resignation spread, thousands of miners, students, and Indians crowded the Plaza de San Francisco near the presidential palace, setting off sticks of dynamite and shouting anti-government slogans.

A few units of the Bolivian police dispatched to the Presidential Palace were disarmed by armed miners and students.

The President Resigns

Sanchez de Lozada, 73, submitted his resignation in a letter to Congress, said a presidential aide, speaking on condition of anonymity. Sanchez de Lozada then reportedly left the presidential residence in a helicopter for the western city of Santa Cruz. Radio reports said he would eventually travel to the United States, but that could not be independently confirmed.

The resignation came after tens of thousands of Bolivians marched through La Paz for a fifth straight day Friday, demanding Sanchez de Lozada step down 14 months into his second term. Columns of students, Indians and miners brandishing sticks of dynamite threaded past street barricades, shouting, "We will not stop until he's gone!"

A virtual general strike has paralyzed Bolivia the last 72 hours. Armed peasants are taking over small towns and police stations in the countryside and all major universities were occupied by left wing students.

With chaos in the streets, military planes airlifted hundreds of stranded foreigners from Bolivia's capital. The US government dispatched several planes to help evacuate American diplomats, their families and business people caught in the revolutionary events at their luxurious hotels in La Paz.

The U.S. military also dispatched an assessment team to Bolivia on Friday to determine if plans need to be updated for protecting or evacuating the American embassy, a military spokesman said. Near the embassy, scores of demonstrators are mounting guard but the diplomatic building suffered no attacks. In fact, many leaders of the revolt asked people not to play in the hands of reaction by attacking foreign targets such as embassies.

The team of fewer than six military experts will assess the situation on Bolivia's streets and recommend possible changes to the embassy's evacuation and protection plans, said Army Lt. Col. Bill Costello, a spokesman for U.S. Southern Command.

On Thursday, the U.S. State Department warned Americans to defer travel to Bolivia. The advisory was considered moot since no airline maintained regular services for the last three days.

The origin of this crisis

The popular outrage against the president was sparked by a controversial proposal to export gas to the United States and Mexico through neighboring Chile.

The proposal tapped deep discord with Bolivia's decade-old free-market experiment, which has failed to narrow the enormous gap between rich and poor in this impoverished country.

The proposal also underscored spreading popular distrust with his administration's U.S.-backed anti-coca growing policies, which have deprived thousands of poor Indian farmers of their livelihood and plunged the president's popularity ratings into the single-digits.

Sanchez de Lozada temporarily suspended the gas export plan last week in the face of a massive street revolts with barricades and tumultuous demonstrations, which human rights groups said claimed as many as 65 lives. But the demonstrations for his resignation continued.

Late Wednesday, the president sought to defuse the growing crisis with a nationally televised address in which he offered to hold a national referendum vote over the plan. But opponents rejected that offer.

In defending the gas export plan, the president called the gas resources "a gift from God" that would bring millions of dollars annually to a cash-strapped Andean country. But few here believe his claims that average Bolivians, many of whom earn only a few dollars a day, would benefit.

Bolivia, which declared its independence from Spain in 1825, is a majority indigenous country where many speak Spanish haltingly. The country yielded its vast mineral wealth to its colonial rulers — and many see the gas-export project as a return to that legacy.

Bolivia also has a long tradition of revolutionary upsurges since the early 50s which included the establishment of workers' councils and workers' and peasants' militias.

Opponents to the export plans also object to the use of Chile, a longtime hostile neighbor, to export the fuel and argued the $5 billion project would only benefit wealthy elites.

The president's increasingly fragile coalition suffered a key blow Friday when Manfred Reyes Villa, a key presidential supporter in Congress, said he was quitting the government after weeks of deadly riots between troops and Bolivian Indians carrying sticks.

"I've come to tell him: 'No more,'" Reyes Villa said. "The people don't believe in this government anymore and there is no other option but for him to resign."

On Thursday, presidential spokesman Mauricio Antezana also resigned.

Reyes Villa's departure left the president isolated as he sought to defuse the crisis in this Andean nation of 8.8 million people — South America's poorest.

A U.S.-educated millionaire, Sanchez de Lozada was president from 1993 to 1997. He took office for a second term in August 2002 after narrowly defeating Evo Morales, a radical congressman and the head of the Movement towards Socialism (MAS).

Morales is one of the most influential leaders in the present revolt.

A weak government plagued by crisis after crisis

During his first days in office last year, Lozada promised to deliver speedy results for a country beaten down by crushing poverty that has ensnared a majority of the country's 9-million population.

He promised new public works programs to create jobs and vowed to improve education for the poor, including the creation of scholarships. An ambitious project to export natural gas was part of the plan to turn around this lagging Andean economy.

But it didn't take long before his government was seriously shaken.

In February, protests over a government austerity plan led to two days of riots that left 31 people dead amid a hailstorm of tear gas in downtown La Paz.

Striking police officers clashed with soldiers in a groundswell of anger over proposed tax increases and salary reductions that sparked demonstrations, widespread looting and the blocking of roads — a traditional Bolivian protest action.

After escaping from the besieged presidential palace, Sanchez de Lozada gave a nationally televised speech appealing for calm and announcing he would suspend the tax increases.

But it laid the foundation for the current crisis, as politicians in his coalition government began to distance themselves from him.

His background also made him the butt of jokes.

Sanchez de Lozada was raised in Washington, D.C., where his father was a diplomat, and he studied philosophy and English literature at the University of Chicago.

He and his wife, Ximena Iturralde, have two grown children, Ignacio and Alejanra, who is a congresswoman.

Sanchez de Lozada speaks Spanish with an American accent, annoying many Bolivians who derisively called him "El Gringo."

"How can we have a president that sounds like that?" asked Jorge Carrasco, a 31-year-old waiter marching in this week's demonstrations.

Sanchez de Lozada's promises to continue Bolivia's unpopular coca eradication — a program the United States has hailed as a major success in the war on drugs — irked many Indian Bolivians, who chew coca leaves to stave off hunger and view the plant as an important part of their centuries-old culture in the Andes.

For days, the main highway link between La Paz and El Alto has been lined with hundreds of demonstrators clutching rocks and sticks and burning barricades. Most countryside roads had been blocked by armed peasants.

Early Friday, a Brazilian air force plane flew 105 people out of Bolivia. Brazilian officials said 53 of those people were Brazilian tourists trapped in La Paz after all commercial flights in and out of the nearby El Alto international airport were halted last weekend.

When the plane arrived in Brazil, it was greeted by an air force band playing the national anthem.

"I felt as if I was in the middle of a war and that I would never be able to return to Brazil," said Antonio Vieira, a school teacher. "Sometimes I watched out the hotel window and saw bodies, some with their heads shattered."

Lana Ferreira, an engineer who lives in Rio de Janeiro, said two buses picked up the group at a hotel and drove them to the airport, where they slept on mattresses provided by the embassy.

Other witnesses paint a picture of certain organization amidst the chaos, describing the formation of neighborhood councils and the opening of miners and peasants provisionary offices in the capital, La Paz.