MEXICO CITY (By Ginger Thompson, NYTimes) July 17, 2006 — For the second time in eight days, thousands of supporters of the leftist presidential candidate, Andrés Manuel López Obrador, filled this city’s historic central plaza to demonstrate their support for his demand for a vote-by-vote recount of Mexico’s disputed July 2 election.

The crowds at this rally — several hundred thousand — were considerably larger than the last and seemed to indicate that the movement started by the embattled former mayor of Mexico City remained strong.

Mr. López Obrador told the throngs of people roaring his name that a recount was not too much to ask to resolve the political crisis that has gripped the nation since election officials declared his conservative opponent, Felipe Calderón, who appeared to be the winner by less than 1 percent of 41 million ballots cast.

That victory has yet to be certified by the Federal Electoral Tribunal as it weighs a legal challenge from Mr. López Obrador. In the meantime, he seems determined to keep up pressure on the tribunal to grant his demand for a recount.

Mr. López Obrador and supporters who helped organize the rally urged his followers to conduct nonviolent acts of civil disobedience, including boycotts of products made by Mexican companies that opposed his candidacy as well as those of some American companies, without explaining why.

He also asked them to stage sit-ins starting Sunday at the 300 district election offices across the country. The purpose, he said, was to prevent any tampering with ballot boxes.

At one point during his speech, Mr. López Obrador smiled as the crowd chanted, “You are not alone.” He told them that the movement he was leading was about more than one man or one political party. He said it was about the future of this country’s fragile democracy.

“I have the deep conviction that despite all the machinery of the state, and all the money of a privileged group, they will not be able to stop the free will of millions of Mexicans,” he said. “That is the greatest force of a democracy.”

Mr. Calderón and his supporters say that their victory is legitimate and that a recount will do more harm than good to an electoral system that was meticulously engineered to move Mexico toward democracy after decades of autocratic, one-party rule.

They have described Mr. López Obrador as so obsessed with power that he will stop at nothing to win, even using mass marches to try to bully his way to power over the decisions of the democratic institutions it has taken Mexico more than a decade to build.

Arturo Sarukhan, a leading adviser to Mr. Calderón, said his candidate would accept a recount if one was ordered by the electoral tribunal. But he said his candidate did not believe a recount was legally necessary, since the votes were counted on election night by citizens recruited at random to be poll workers.

“They are seeking to pressure the tribunal to say this is too complicated, let’s annul the whole thing,” Mr. Sarukhan said. “We are convinced this is not about a recount. This is about annulment.”

In a voluminous complaint before the electoral tribunal, Mr. López Obrador charged that the voting was riddled with mistakes and rigged against him by President Vicente Fox, who openly supported Mr. Calderón, who is from the same party.

He also accused business leaders of meddling in the election by conducting a campaign that depicted Mr. López Obrador as a danger to the political and economic stability Mexico has enjoyed over the last six years.

At 1:30 p.m. Sunday, Mr. López Obrador led about 200,000 supporters down one of the city’s main thoroughfares and into the Zócalo, as the city’s main plaza is known. About 200,000 more were already waiting there.

Rossana Fuentes Berain, a political analyst at the Autonomous Technological Institute of Mexico, looked out on the crowd from the terrace of a hotel adjacent to the plaza and wondered whether Mr. López Obrador could keep a rein on the outrage he had unleashed.

“I am very worried,” she said. “If he can convince so many people that the democratic system isn’t working, then we are going to throw away 20 years of building trust and confidence.”

People interviewed at the rally said their distrust of the system had compelled them to come from as far north as Mexico’s border with the United States and as far south as the state of Chiapas.

Like the crowd of people who came out last week to support Mr. López Obrador, this gathering also seemed to be a broad cross section of people with different levels of education and incomes.

Many of those interviewed brought up the 1988 election, considered a pivotal moment in this country’s long history of electoral fraud. Some said that although there had been progress since then, it had not been enough to stop the entrenched oligarchies from using their powers to stand in the way of the will of the people.

Others said that although they supported Mr. López Obrador, they worried that his language had become increasingly volatile and could push the country into violence.

“People are tired of so much injustice,” said Pablo Huitrón Neguis, 44, an elementary school teacher. “We have fought years to have our votes respected, and now they are using the same old strategies to rob us again. López Obrador knows that if the people unite, there is no one who can stop us.”

There were lots of angry expressions from unlikely places.

“If blood must be spilled, then so be it,” said a housewife, Graciela Saavedra, 44. “We don’t want it, but we are tired of accepting fraud with our arms crossed.”

There seemed to be just as many calls for calm. “We know there was fraud,” said Pablo Serna, 27, a dentist from the north central state of Zacatecas. “But there is no reason to generate so much resentment. There is also no reason to discredit everything because the movement could turn against us.”

Although it was clear Mr. López Obrador would not back down from his fight, his appeals Sunday seemed toned down from earlier this week, when election officials challenged the credibility of several videos he presented as proof of fraud.

“He who owes nothing, fears nothing,” Mr. López Obrador said, explaining why in his view Mr. Calderon should not object to a recount. Later, he added, “The stain of an illegitimate election cannot be cleaned with all the water in all the oceans.”

Antonio Betancourt and James C. McKinley Jr. contributed reporting for this article.