People protest same-sex marriage bill outside Argentina's Congress in Buenos Aires on Tuesday. ((Natacha Pisarenko/Associated Press))

Senators in Argentina are set to vote Wednesday on a bill that would legalize same-sex marriage, but the bill is facing stiff opposition from the Roman Catholic Church and other groups.

Tens of thousands of demonstrators gathered in Buenos Aires Tuesday night to protest the bill, with more protests scheduled for Wednesday.

"In the name of modernizing human rights, what this bill actually does is produce a major step backwards for humanity," said Roman Catholic Bishop Antonio Marino, one of the church's leading voices opposing the bill said.

Notes from the field "There is a lot of pressure on these senators and a lot of indecision," CBC's Connie Watson said from Buenos Aires, adding that some senators may choose to skip the vote because the measure has prompted such heated debate. "Buenos Aires is considered one of the most gay-friendly cities in Latin America. There are civil unions allowed in this city — but it's not on the national level," she said. "This would allow gay marriages on the same level as heterosexual marriages and allow adoption, and those are the things that really concern the people against this vote." Tens of thousands of people attended Tuesday night's massive demonstration against the bill, Watson said, with smaller crowds turning out for rallies in support of the bill.

"If you want to talk about progress, the only progress this brings is towards decadence."

Mercedes Riglos, who attended a demonstration against the new measures, said she was opposed to marriage between two people of the same sex, adding that "we don't want them to adopt children."

Supporters rally

Supporters of same-sex marriage also took to the streets, hosting loud rallies in the capital and other cities.

Andrea Dati, the leader of a women's rights group, attended a gathering Wednesday in support of the proposed law.

"We are here to show our support for the bill because the senators are under a lot of pressure from the Catholic Church to vote against it," she said.

President Cristina Fernandez has promised not to veto the measure, which was approved by the lower chamber in May.

If the bill becomes law, Argentina would become the first country in Latin America to legalize same-sex marriage.

Mexico City has legalized same-sex marriage, allowing couples to adopt children, apply for bank loans together, inherit wealth and be included in the insurance policies of their spouses.