Hundreds of thousands of demonstrators thronged Mexico City's central square and surrounding streets on Sunday to protest the federal prosecution of the capital city's mayor, a leading contender for president in 2006.

Their mouths covered with white masks, protesters in the so-called "March of Silence" filled 12 lanes of the Paseo de La Reforma (search) in columns that stretched for several miles in a demonstration of support for Mayor Andres Manuel Lopez Obrador (search).

They carried signs with the words, "Lopez Obrador, we are with you" and said the legal case against the mayor is testing their faith in Mexico's government.

"There's an aggression underway against democracy in Mexico," said Carlos Gonzalez, 48, a builder from the central Mexican state of Puebla. "We're defending what we have fought for."

Lopez Obrador is stuck in legal limbo after Congress voted on April 7 to strip him of the immunity from prosecution to face charges in an obscure land expropriation case.

Federal authorities allege the mayor ignored a court order to stop construction of a hospital access road. Lopez Obrador claims the charges were drawn up by President Vicente Fox's (search) administration and other political rivals to keep him out of next year's race, which he leads in all public-opinion polls. Fox has denied the allegations.

On Friday, a Mexican judge sent charges of abuse of authority against Lopez Obrador back to prosecutors, citing a procedural mistake in setting bail. The attorney general's office has said it would refile the charges.

Under most interpretations of Mexican law, anyone facing criminal charges cannot run for public office.

Addressing Sunday's massive gathering, Lopez Obrador mixed politicking for higher office with decrying the alleged plot against him.

"The only logical explanation I can find for these interrogations is that our adversaries have an irrational fear of what we are proposing," he said.

Much of the mayor's popularity stems from new spending on government funded construction and social programs, including pensions for the elderly.

He outlined similar proposals on a national scale before a sea of protesters on Sunday.

"The proposal we have is to establish in our country a state of well-being, a state of equality and fraternity, in which the poor, the weak and the forgotten find protection against economic uncertainties, social inequalities," he said. "There are sufficient resources."

Signs along the protest route derided Fox, whose election in 2000 ended 71 years of rule in Mexico by the Institutional Revolutionary Party (search).

The mayor's only rival for the nomination of the left-center Democratic Revolution Party (search) — party founder Cuauhtemoc Cardenas (search) — even joined the march.

Lopez Obrador repeated vows to return to his office on Monday after a week of campaigning outside the city, despite suggestions that such an action could lead to more charges against him.

Prosecutors contend that the April 7 vote by Congress immediately disqualified him from continuing in office.