By GINGER THOMPSON and JAMES C. McKINLEY Jr.
 
MEXICO CITY, April 7 - With the fate of the Mexican presidency at stake, hundreds of thousands of people thronged this city's central plaza today to support the leftist Mayor Andrés Manuel López Obrador, who faced a vote in Congress that could force him off the ballots in next year's national elections.

The lower chamber of deputies today was scheduled to approve a measure that would strip Mayor López of his official immunity so that he could stand trial in a minor land dispute. And since the Mexican Constitution holds suspects guilty until proven innocent, Mr. López would be banned from politics until the end of the trial.

In addressing the immense crowd today, the 51-year-old Mr. López called the proceedings against him a "farce" staged from the offices of Vicente Fox, Mexico's first opposition president. He charged that the attempt to knock him out of the race for president would undermine country's fragile democracy, moving Mexico back into a past when the political elite ruled like monarchs.

"The move to prosecute me," he said, "returns Mexico to authoritarian times when Los Pinos decided who would or would not become president." Los Pinos is Mexico's presidential mansion.

Mr. López added that President Fox's National Action Party and the Institutional Revolutionary Party, which ran this country for seven consecutive decades, had forged an unlikely alliance to cripple his left-wing movement and maintain the status quo.

"Whichever of them wins, things remain the same," he said. "They maintain a corrupt and privileged regime, and will continue devouring the country."

The case against the mayor has polarized the country into two angry camps, raising concerns about civil unrest from here to Wall Street. Mr. López's spending on social programs and extravagant public works projects has made him popular among this country's poor masses and its struggling middle class. Meanwhile his talk against free trade policies, of renegotiating the national debt and substantial increases in social spending has brought comparisons of Hugo Chávez from the wealthy business classes.

But no matter what side they were on, Mexican political leaders, intellectuals and business executives have said they considered this a pivotal moment in their history that transcends law and order. At stake, they said, was not whether or not Mr. López committed a minor crime, but the legitimacy of the multiparty democracy that emerged when Mr. Fox broke the P.R.I.'s hold on power five years ago. Some also worried that the case against Mr. López further eroded public confidence in the government and would taint the mandate of this country's next president.

Mr. López, who has a history of leading violent protests, called on followers this time to mount a peaceful campaign of civil disobedience and avoid giving into provocation that could lose their movement the support of the majority of the people. He made clear that he wanted this movement to resemble those led by the Rev. Dr. Martin Luther King Jr. in the United States and Gandhi in India. And he added, that he was willing to go to jail for his beliefs.

"Nothing of violence," he ordered the crowds. "No falling to provocation. This movement has been and will be peaceful. To do otherwise would be to act in the logic of our adversaries, and we cannot allow that."