The law allows women in the Federal District - Mexico City´s legal entity - to interrupt a pregnancy any time in the first 12 weeks. Public and private hospitals will be required to offer the procedure.

"This is history," said Raffaela Schiavon, executive director of Ipas Mexico, one of several reproductive rights organizations that have lobbied for legalization for many years. "This is an important victory for women´s health and rights, but it is only the beginning."

Outside the Federal District, no state allows abortion except under exceptional circumstances, such as rape. But pro- choice sentiment runs strong in the capital, which has been governed by the Democratic Revolution Party (PRD) since 1997.

That sentiment was reflected in the vote Tuesday, with 46 deputies in the 66-member Federal District Legislative Assembly (ALDF) voting in favor of the legalization measure.

There were 19 nay votes, including all 17 deputies from President Calderón´s National Action Party (PAN).

There was still legislative work to be done after the vote, which came after more than five hours of debate. Though the measure was approved in overall terms, deputies from several different parties chose to "reserve" certain segments of the legislation for separate discussion.

The pending votes on those specifics were not expected to change the outcome.

PAN deputies and party leaders said they would initiate a constitutional challenge of the new law.

With only two allies from the Green Party voting with them, the 17 PAN deputies probably won´t be able to gather the required 22 signatures to mount the challenge themselves. They say they will petition the Federal District Human Rights Commission (CDH-DF) to bring the law to the Supreme Court.

"We´re going to use legal arguments to convince (CDH-DF president) Dr. Emilio Álvarez Icaza to present it before the court," said Mariana Gómez del Campo, the PAN party leader in the Federal District. "This law contradicts the Constitution."

There has been no indication from the CDH-DF that such a move would be considered.

Tuesday´s vote came after several months of aggressive protest by abortion proponents, led by the PAN, the Catholic Church hierarchy and ultra-conservative organizations such as Pro Vida.

Hundreds of demonstrators from both sides of the issue assembled in the Historic Center and alongside Alameda Park, occasionally shouting at each other but otherwise avoiding incidents.

More than 500 uniformed officers kept the public from approaching within two blocks of the ALDF chambers.