Mexico's federal government has challenged a gay marriage law approved by lawmakers in Mexico City, the Mexico City-based Excelsior reported.

The law – approved in December and expected to take effect in March – is a first for Latin America. It gives gay and lesbian couples all the rights and responsibilities of marriage, including the right to adopt children. Previously, the city government recognized gay couples with civil unions, but gay adoption was banned.

The federal Attorney General's Office has challenged the law on the basis that it violates the constitution. The office has asked Mexico's Supreme Court to void the law because it “strays from the responsibility of the government to place a priority on safeguarding the interests of children.”

Federal lawyers also argue that the city's civil union law grants gay couples virtually the same rights as marriage, except those related to children.

City officials, however, said the action would not interfere with the start of the law, scheduled to take effect on March 4.

In an interview with the Excelsior, Leticia Bonifaz, the city's legal adviser, said she was “totally confident that this is an issue of fundamental rights ...”

The Roman Catholic Church and the conservative PAN party have widely criticized the law. Mexico's Roman Catholic archbishop, Cardinal Noberto Rivera Carrera, has called the law “immoral” and “reprehensible.” PAN leaders have mounted a campaign to overturn the law.

Mexico City is the nation's seat of government and also its largest city. Nearly 10% of Mexicans call the city home.

A recent poll found 46% of residents approve of the law, 43% oppose it, and 11% are undecided.