By Catherine Bremer

MEXICO CITY, April 26 (Reuters) - A group of Catholic lawyers will complain to an international human rights court next week about the legalization of abortion in Mexico City, a Roman Catholic Church spokesman said on Thursday.

The abortion law passed easily in the capital's leftist-controlled assembly on Tuesday, angering many Catholics who see it as the latest shift away from Christian values in the world's second-biggest Catholic country.

"The College of Catholic Lawyers, a secular association of believers, is going to start an international campaign to take this case to the Inter-American Court of Human Rights ... to show the world the lack of democracy in Mexico City," said Father Hugo Valdemar, spokesman for the Archdiocese of Mexico City.

The lawyers plan to take their complaint on Wednesday to the Costa Rica-based rights court, an arm of the Organization of American States.

Valdemar said the abortion law goes against a clause in Mexico's constitution that says the state must defend human life "from conception until its natural end."

Mexican law bars the church from taking the case to the Supreme Court, and the measure's opponents in the Mexico City assembly are too weak to launch an appeal.

In Latin America, only Cuba, Guyana and U.S. commonwealth Puerto Rico allow abortion on demand. Some countries permit it in special cases, such as after rape.

The Mexican law, effective in the coming days, means women in the capital can abort in the first three months of pregnancy. A ban remains elsewhere in the country.

Opinion polls show people in Mexico City divided over abortion, but nationally seven in 10 are opposed to it. Around 90 percent of Mexicans say they are Catholic.

The abortion law follows the legalizing of gay civil union in the capital and one other state. Mexico City lawmakers have also pushed for a euthanasia law.

Valdemar said Cardinal Norberto Rivera, Mexico's top clergyman, will lash out after Mass on Sunday at the abortion law, which Pope Benedict had urged Mexican bishops to fight.

"Mexican culture is profoundly rooted in Christianity," Valdemar said. "The church feels a pastoral right to defend life from the time it starts and to raise its voice and oppose these kind of perverse laws."

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