MEXICO CITY — Mexico City lawmakers voted to legalize abortion during the first three months of pregnancy, a landmark decision likely to heighten church-state tensions in the Roman Catholic nation and lead to a bitter court battle.

Abortion-rights advocates said they hoped the vote would be the start of a new trend across Mexico and other parts of Latin America, where only Cuba and Guyana permit women to have abortions on demand in the first trimester. Most other Latin American countries allow it only in cases of rape or when the woman's life is at risk. Nicaragua, El Salvador and Chile ban it completely.

But the debate in Mexico appeared far from over. Opponents vowed to challenge the law before the Supreme Court, saying it violates individual rights.

The church has played a vocal role in opposing the measure, a position shared by President Felipe Calderon's conservative National Action Party.

The bill, approved 46-19, with one abstention, will take effect with the expected signing by the city's leftist mayor.

The procedure will be almost free for poor or uninsured city residents.