Mexican federal prosecutors announced Wednesday that they will try to overturn Mexico City's gay marriage law, which allows same-sex couples to adopt children, on the grounds it violates the constitution.
The Mexico City law, approved in December and due to take effect in March, is the first of its kind in Latin America.
The federal Attorney General's Office said in a statement that it believes the law "violates the principle of legality, because it strays from the constitutional principle of protecting the family."
It cited an article in Mexico's constitution that suggests - but does not state - that the framers envisioned families constituted by men, women and children. The article states: "Men and women are equal before the law. This protects the organization and development of the family."
The office said it filed an appeal with the country's Supreme Court asking it to void the law, arguing it also "strays from the responsibility of the government to place a priority on safeguarding the interests of children."
The Roman Catholic Church in Mexico has harshly criticized the law, and President Felipe Calderon's conservative National Action Party has mounted a campaign against the measure.
Mexico City legislators argued the law simply gives same-sex couples the rights that heterosexual couples have regarding social security and other benefits.
Legislators said there is no evidence children adopted by same-sex couples suffer any disadvantages.
The federal prosecutor's office previously challenged a Mexico City law legalizing abortion, but the Supreme Court upheld the measure in 2008.
An Argentine couple participated in Latin America's first gay wedding in December, but interpretations vary on whether the law allows same-sex unions in Argentina and the question is now before that country's supreme court.
Argentina's constitution is silent on whether marriage must be between a man and a woman, effectively leaving the matter to provincial officials, who approved the wedding. But a law specifically legalizing gay marriage has been stalled in its Congress since October.