Mexico's largest leftist party vowed Monday to defend a landmark Mexico City abortion law with street protests and political pressure in the face of attempts by the conservative federal government to overturn it in court.

The Democratic Revolution Party, which holds power in the capital, called its supporters to block federal government offices later this week in defense of the law, which legalized abortions in the first 12 weeks of pregnancy.

Elsewhere in Mexico, abortion is only allowed in cases of rape, when the mother's life is in danger or if the fetus has severe fetal defects.

On Friday, the federal Attorney General's Office and National Human Right's Commission filed legal challenges with the Supreme Court, arguing the Mexico City law violates a constitutional clause that guarantees the right to life and that city lawmakers cannot legally approve measures related to health.

"It's a political maneuver to satisfy a certain public opinion over this law," said Mexico City's leftist mayor, Marcelo Ebrard. "But legally, it's got no base."

The law's approval by Mexico City's assembly was bitterly opposed by conservative politicians and the Roman Catholic Church. Mexican bishops even argued that lawmakers who voted for the bill were excommunicating themselves from the church.

About 90 percent of Mexicans say they are Roman Catholic, and President Felipe Calderon, of the conservative National Action Party, has spoken out publicly against abortion.

The abortion law is part of a package of socially liberal measures being passed by Mexico City's leftist assembly. Earlier this year, lawmakers approved gay civil unions, and they are debating a bill on euthanasia.

The only other Latin American countries that allow abortion are Cuba and Guyana.