"We are requesting a ballot-by-ballot recount," said leftist Andres Manuel Lopez Obrador, drawing rousing cheers from the crowd that packed Mexico City's huge downtown Zocalo square and neighboring streets.

"We are certain we triumphed on July 2," he said, claiming widespread irregularities marked the electoral process that gave victory by a razor-thin margin to conservative Felipe Calderon, of the governing National Action Party (PAN.)

He called for nationwide marches that will start Wednesday and converge on the capital, and stressed the protests would remain peaceful.

"There is clear evidence that they took away our votes to favor the right," said Lopez Obrador, a former Mexico City mayor who advocates the cause of the millions of impoverished Mexicans.

"It was a massive fraud, those right-wingers sure know how to cheat," Angel Farfan, 60, one of his supporters said before joining the rest of the crowd in chanting "Obrador, president."

Lopez Obrador claimed that President Vicente Fox illegally campaigned in favor of Calderon, that the PAN had far exceeded campaign spending limits and that the governing party bought votes.

The lawyers will present their challenge to the Federal Electoral Tribunal on Sunday, and will also file complaints with the Supreme Court, which can issue recommendations to the tribunal, the final arbiter of electoral disputes.

Lopez Obrador lashed out at Calderon, claiming the Harvard-trained conservative served "a very powerful interest group in Mexico that for quite a while has converted the government into a committee at the service of a minority."

"This group not only has no interest in improving the life of millions of Mexicans, but has profound contempt for the poor," Lopez Obrador said at a news conference before the rally.

Asked about foreign leaders, including US President George W. Bush, who congratulated Calderon on his victory, Lopez Obrador insisted "there is no president-elect in Mexico."

The electoral tribunal has until September 6 to render its final verdict on the outcome of the July 2 election.

Calderon, 43, who won with an advantage of just over 244,000 votes, or 0.6 percentage points, dismissed claims of fraud during the electoral process, which a number of international observers called free and fair.

He said that now was the time for Mexicans to put the campaign disputes behind them and work to create a politically stable country attractive to foreign investors.

Speaking to journalists on Friday, Calderon said that battling poverty, which affects half the 103 million population, would be one of his top priorities.

He urged the United States to invest in economically depressed areas of Mexico to create badly-needed jobs.

This he said would be far more effective in halting illegal migration to the United States than a wall the US administration plans to build along the border. Last year, an estimated 400,000 Mexicans crossed the border illegally, often risking their lives trekking along deserts and arid mountains.

The next president will have to deal with a divided Congress, after the PAN got the most votes for the House and the Senate but fell well short of an outright majority.