Led by Norberto Rivera, the Mexico City Cardinal, the march covered 25 blocks to the Basilica of the Virgin of Guadalupe, Mexico's patron saint, where the cardinal celebrated an afternoon Mass on a balcony overlooking the basilica's packed main plaza.
"We are united here so that they hear our voice, the voice of life," Rivera, who regularly comments on politics despite a constitutional ban on such activity by clerics, told an applauding crowd.
Summoned by Mexico's Roman Catholic Church, Sunday's so-called "pilgrimage for life" included extended families, Catholic youth groups and nuns who waved banners and balloons emblazoned with the image of the Virgin of Guadalupe.
Some wore white to symbolise purity, and recited the rosary as they walked alongside pickup trucks equipped with loudspeakers that blasted hymns and prayers. Others carried signs reading "Let's defend life."
The event coincided with an unrelated march by supporters of Andres Manuel Lopez Obrador, the former left-wing presidential candidate, who has refused to accept his narrow loss to President Felipe Calderon last year and has established a "parallel" government.
In an echo of the vitriolic election campaign, the abortion debate pits Lopez Obrador's Democratic Revolution party (PRD) - which proposed the legalisation measure - against Calderon's conservative National Action party, which opposes it.
Privatisation of oil
Lopez Obrador did not broach the subject during his appearance in the capital's main plaza, the Zocalo. He stuck to establishing policies for his alternative political movement, including opposing privatisation of the national oil monopoly Pemex - something he also accuses Calderon of planning, although the president has denied it.
Colombian Cardinal Alfonso Lopez Trujillo, the Vatican's top anti-abortion campaigner, who was in the capital for the Third International Pro-Life Congress, was not visibly present at Rivera's Mass.
"We need to stop thousands of women from dying in unsafe operations"
Senator Carlos, Navarrete
The Mexican constitution bars foreigners - including Lopez Trujillo and members of US anti-abortion groups currently attending the conference - from political activism.
In 2000, authorities had barred US and Canadian anti-abortion activists from returning to Mexico for five years after they joined protests in Mexico City. Such groups were not noticeable at the march.
Mexican law prohibits political involvement by domestic religious leaders, although that provision has been weakly enforced - especially under the church-friendly PAN. In his sermon on Sunday, Rivera said the church's fight against abortion is not about politics, but about the moral teachings of God.
The PRD argues that current Mexican law forces poor women to seek back-street operations, while the wealthy can travel to the US.
"We need to stop thousands of women from dying in unsafe operations," said Senator Carlos Navarrete.
The measure is expected to pass easily in Mexico City, a federal district with a PRD-dominated legislature that recently approved same-sex civil unions in the capital; but it will face a tougher passage at the federal level.