MEXICO CITY -- It is a sight not seen in perhaps 140 years: an open-air Mass by the Roman Catholic Church attended by tens of thousands at the Zocalo, a sprawling plaza in the heart of Mexico.

The celebration Saturday is yet another sign that the long and sometimes bloody hostility between the government and church in Mexico has broken down -- the church is being allowed a more public role.  In a March 25 pastoral letter, Mexico's bishops called for "a reformulation of the whole political system," including constitutional reforms.  The leading opposition candidate for president, Vicente Fox, has pushed the limits of the church's new role -- once by displaying a banner of the Virgin of Guadalupe, Mexico's patron saint, at a campaign rally.

Church and state have sparred in the press over just how far priests can go to influence politics under a 1992 reform that ended many of the exceedingly harsh and very severe laws imposed on clergy since 1855.  Those restrictions, meant to separate church from state, included an 1873 ban on outdoor religious ceremonies.  The Zocalo has not seen an open-air Mass since before that ban was imposed.

Saturday night's Mass was the highlight of the National Eucharistic Congress, gathering all the nation's bishops and a Vatican representative.  "The new church cannot be the church of silence," Cardinal Norberto Rivera said as he opened Saturday's Mass, referring to more than a century of state restrictions on religion.

The event was deeply meaningful for Erasmo Chavez, a 70 year old church lay worker with an image of the Virgin of Guadalupe pinned to his sweater. "When I was a child, we had to come secretly to celebrate services," he said, peering towards the temporary altar set up before the capital's cathedral.

With 100 million people, Mexico is one of the most populous predominantly Catholic nations in the world.  The Pope was greeted with raucous fervor on visits to Mexico in 1979, 1990, 1993 and 1999.

The church played a major part in political and economic life for about 350 years. In the 18th century, however, 'reformers' attacked the church's privileges and land holdings.  The Constitution of 1857 separated church and state, and other laws removed marriage and divorce from church jurisdiction. Other laws forbade church schools and religious orders.