Hundreds of Mormons are expected to mail letters resigning from the faith after gathering in Salt Lake City on Saturday to protest a new church policy that calls married same-sex couples apostates and bars their children from baptism.

The Church of Jesus Christ of Latter-day Saints has long opposed same-sex marriage, but has angered some members with a new directive on how it should deal with Mormon families headed by wedded gay and lesbian couples.

The policy, approved last week by leaders of the church, which claims more than 15 million adherents worldwide, added same-sex marriage to the list of acts considered to be a renunciation of the faith and thus subject to church discipline, including excommunication.

It also prohibits natural or adopted children of gay married couples from being baptized in the faith until they turn 18, leave their parents' home and personally disavow same-sex marriage or cohabitation.

Church leaders elaborated on the policy on Friday, saying the withholding of baptism would apply only to children whose primary residence is with a same-sex couple.

For children who have already been baptized, the provisions do not curtail their membership activities of priesthood privileges, the leaders said.

At least 750 demonstrators will gather on Saturday afternoon at a Salt Lake City park across the street from the headquarters of the church, organizer Lauren Elise McNamara said on Friday.

Most of them will collectively mail their letters of resignation there, while hundreds more are expected to mail them from elsewhere, she said.

Because resigning from the church is a complicated process, an attorney will be on hand to help with the paperwork, said McNamara, who converted to Mormonism as an adult and plans to resign along with her husband.

Critics of the church say its policy targeting the children of same-sex couples is unfair, and that it is bound to set some family members against each other.

The policy follows a landmark ruling in June by the U.S. Supreme Court that legalized same-sex marriage.

"We don't want to see anyone leave the Church, especially people who have been struggling with any aspect of their life," Eric Hawkins, a spokesman for the church, said in an email.

The church this year announced support for U.S. laws protecting gays and lesbians from discrimination in housing and employment.