Leftist Andres Manuel Lopez Obrador swore himself in as Mexico's "legitimate" president Monday, launching a parallel government he hopes will prevent President-elect Felipe Calderon from governing.

The ceremony is the latest chapter in Lopez Obrador's unsuccessful battle for the presidency. He claims fraud and dirty campaign tactics were responsible for Calderon's narrow victory in the July 2 vote, and his parallel government could spend the next six years calling for street protests that have already dented the economy and prompted travel warnings from the U.S. Embassy.

Rosario Ibarra, a human rights activist and senator for Lopez Obrador's coalition, placed the presidential sash across his shoulders during Monday's ceremony. While the action lacks legal recognition, Lopez Obrador hopes to assume the moral leadership of millions of poor Mexicans.

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"I pledge ... to serve loyally and patriotically as legitimate president of Mexico," Lopez Obrador said. "I pledge to protect the rights of Mexicans and to defend Mexico's sovereignty and patrimony, and ensure the happiness and welfare of the people."

Speaking before about 100,000 supporters, he vowed to draw up a new constitution to oppose the building of U.S. border fences and limit the power of big corporations, the media and the "neo-fascist oligarchy" he claimed had taken over the government.

Based in Mexico City, Lopez Obrador's parallel government has its own Cabinet, but it will not collect taxes or make laws and will rely on donations to carry out its plans.

Lopez Obrador said he plans to spend three days a week in Mexico City and spend the other four days touring Mexico "to create the most important citizens' organization in all our history." But his movement's first action will be to try to prevent Calderon's Dec. 1 inauguration ceremony.

"We are going to make Calderon realize at all times that he is an illegitimate leader," said 55-year-old Lopez Obrador supporter Beatriz Zuniga, an unemployed professor of Latin American studies. "He's got a limited amount of time. This man will not finish his term."

Supporters carried signs lashing out against not only Calderon, but a variety of foes they say had lined up to marginalize Lopez Obrador: the Roman Catholic Church, the mainstream media and even rival leftists such as Zapatista rebel leader Subcomandante Marcos.

But some members of Lopez Obrador's Democratic Revolutionary Party, or PRD, have already expressed disagreement with his strategy of using Congress — where the PRD is now the second-largest party — as an arena for protests rather than negotiations. Lopez Obrador pledged Monday to make more legislative proposals.

Some residents also have tired of Lopez Obrador's brand of political activism, which includes endorsing the leftist protesters who seized Oaxaca city for five months to demand the resignation of the state's governor. Lopez Obrador's own supporters blocked Mexico City's main boulevard for nearly two months this summer.

"This affects the country's image," said Marco Ramirez, 34, a university researcher watching the protesters from a sidewalk cafe. "It puts out a very bad image."

Columnist Armando Fuentes described Lopez Obrador's "swearing-in" ceremony as "a circus act, a farce" in the newspaper Reforma.

But Oscar Aguilar, a political science professor at Mexico City's Iberoamerican University, said "the social and political conditions (in Mexico) are fertile ground for this kind of leadership."

"Many of the poor ... see this type of leadership as a solution," he said.

Dissatisfaction is already bubbling up in many places. Mexico City was rattled earlier this month when several bombs exploded at political offices and banks. Small radical groups not tied to Lopez Obrador claimed responsibility for the blasts, which injured no one.

But the political unrest has affected tourism, one of the country's main sources of income, which was down 4.3 percent in the first nine months of 2006, compared to the same period in 2005.

President Vicente Fox canceled a traditional Nov. 20 parade commemorating the country's 1910-1917 Revolution, apparently to avoid friction with Lopez Obrador's supporters.

Some of Lopez Obrador's closest aides have suggested they will follow Bolivia's example and try to use protests to force Calderon from office, as demonstrators did with a succession of leaders there. Lopez Obrador has not ruled that out.

The business-friendly Calderon, of Fox's conservative National Action party, may also move quickly to win over disaffected Mexicans. He has already borrowed some ideas from Lopez Obrador's agenda, such as pensions for the elderly and reduced utility rates for the poor.

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