The Senate action, a precursor to an expected final approval of the complete reform that could happen as early as Wednesday, came just hours after a majority refused to delay consideration of the reform.

Legislators in both houses have been criticized by reform opponents, and even by some supporters, for pushing the historic reform through the legislative process at warp speed.

"This issue has never been analyzed in depth," said Ricardo Monreal, a member of the Democratic Revolution Party (PRD) that voted for a delay and against the reform. "Nor have any calculations been done on what these new rules will mean for workers´ pensions."

President Felipe Calderón, however, has argued that quick passage of the reform is needed to save the government workers institute, known as ISSSTE, from bankruptcy.

Rising life expectancy and an increasing imbalance between new and retiring workers has put so much pressure on the system that it has to be subsidized from the general government coffers, administration officials claim.

The president´s National Action Party (PAN) teamed with the Institutional Revolutionary Party (PRI) to defeat the delay attempt and pass the general reform.

Reform opponents, frustrated by their inability to influence the process, marched on the Senate building in Mexico City´s Historic Center from the early morning on into the evening. Tuesday´s protest was larger than those during the Chamber of Deputies consideration of the bill last week, but less rowdy.

At various points during the day, marchers closed sections of Insurgentes, the Eje Central as well as several Historic Center streets surrounding the Senate building.

Security around the Senate was heavy throughout the day, with federal police backing up city police at all entry points, and reporters denied admission without special badges.

The protest marches were led by the nation´s biggest unions, including those representing university workers, telecommunications employees, and the umbrella workers organization known as the National Workers Union, or UNT.

The clashes over the ISSSTE reforms have been particularly intense because it represents the first showdown between Calderón´s private market preferences and Mexico´s traditional shared-pool approach to public employees´ retirement benefits.

More specifically, labor leaders worry that government employees will see less money late in life as their retirement plans are subjected to the whims of the market.

"This law is regressive," said Javier González Garza, leader of the PRD deputies. "Not even the United States has a system like the one they´re trying to implement."

Unions leaders threatened to call a general nationwide work stoppage if the Senate approves the reform, as expected.

EL UNIVERSAL staff writers Arturo Zárate, Alberto Morales and Ricardo Gómez contributed to this report.