MEXICO CITY, Mexico — Inside Mexico City’s spectacular metropolitan cathedral, Cardinal Norberto Rivera conducted his last Sunday Mass of the year to packed benches.

But rather than focusing on the goodwill of the holiday season, Rivera’s sermon centered on what he said was a new affront to the nation’s Roman Catholic values: gay marriage.

Calling the unions “perverse” and an “aberration,” the prelate called on his flock to fight against a new law allowing gay marriage in Mexico City — the first of its kind in Latin America.

“This perverse example cannot spread. It is necessary to constitutionally defend the family,” Rivera said in his characteristic deep booming voice. The law “is unjust, inadmissible and condemnable.”

A wave of social liberalization has transformed the Mexican capital in recent years, but the gay marriage act has ignited a far more negative reaction than other reform measures. The law, which was approved by Mexico City’s assembly on Dec. 21 and signed into law Tuesday, also allows gay couples to adopt children.

The comments by Rivera — the foremost Catholic cleric in Mexico — are part of a series of harsh attacks and calls to action against the act.

Mexico’s College of Catholic Lawyers on Monday went even further than Rivera, calling on the faithful to try to stop gay marriages in the city.

“We need a pacific resistance so that no [gay] couple marries in this capital and the definition of marriage does not change from its natural meaning,” said college president Armando Martinez.

In the neighboring city of Ecatepec, Bishop Onesimo Cespeda was more blunt, simply saying that the idea of gay marriage was “stupidity.”

The clash between the church and state over the issue highlights the broader culture wars being fought south of Rio Grande.

Mexico has long been influenced by a conservative Roman Catholic hierarchy and almost 90 percent of the population purport to the faith.

But the nation has also borne an influential leftist tradition since the days of the Mexican Revolution at the dawn of the 20th century.

The leftist Democratic Revolution Party (PRD) has controlled Mexico City since 1997, and passed a wave of other reforms, making the capital into what advocates say is a beacon of social progressiveness.

The changes have been possible because of Mexico's federal system, which gives the capital's assembly the power to pass local laws.

In 2007, the assembly approved same-sex civil unions as well as allowing abortion in the first 12 weeks of any pregnancy. The following year, it approved a limited form of euthanasia.

The gay marriage law may have been a surprise in much of the world, but to Mexico City residents it was the latest in a reformist agenda they have become accustomed to.

However, the religious reaction to the latest act has included a far more fiery rhetoric than to the other laws. The church was relatively quiet in criticizing the civil unions while the attacks on abortion did not include discriminatory language.

In going after gay marriage, however, it seems to be taking a harder stand to finally push back against the reforms.

“The level of discrimination and homophobia from the church has been surprising,” said Lol Kin Castaneda, a prominent gay activist who has been campaigning for legalized gay marriage for 31 years. “They have been making very irresponsible statements, which polarize society and could provoke violence.”

PRD lawmakers also alleged that church officials are actually breaking the law by stepping so far into the debate. The Mexican Constitution defines the nation as secular and prohibits religious officials from involvement in politics.

On Tuesday, PRD legislators sent the federal government a letter demanding it clamps down on the church over the outbursts. The letter highlighted a meeting between Roman Catholic and Evangelical ministers in which they discussed tactics to combat the new law.

“The fact that ministers from different religions meet in a church with political ends to attack civil laws seems to be a very serious matter,” PRD assembly woman Maricela Contreras told reporters.

The church’s stance comes while the political right has been relatively quiet on the landmark legislation.

The ruling National Action Party of President Felipe Calderon includes many social conservatives with deep religious faith. But National Action politicians shy away from commenting on the culture wars, seeing it as unpopular with their electoral base in the urban middle class.

On the streets of Mexico City, opinions are very mixed and divided on the law.

Many say they have no objection to gay marriage itself, but it is common to hear opposition to the idea of adoption.

“If two men or women want to be together that is fine with me. That is their right. But why bring a child into it?” said Mauricio Fernandez, a 45-year-old business owner. “A baby has the right to a mother and a father.”

Activists such as Castaneda retort that opponents are highlighting the adoption element to distort the debate. Mexican adoption laws never specifically stated that couples had to be heterosexual anyway, she says. Furthermore, this should not get in the way of gay couples getting their rights.

“There will be a struggle ahead but we will win this argument,” she said. “The church’s slandering shows a cry of desperation.”