The reactions of those gathered outside the congress chamber clearly signalled which way the vote had gone. On one side, gay rights activists sang, cheered and waved rainbow banners. Across the hall, crestfallen Catholics clutched crucifixes and prayed.

Argentina's lower house passed a gay marriage bill last week in a historic vote that could add a Latin American country to the short list of countries that allow homosexual couples to wed.

"Love isn't owned by heterosexuals," Felipe Sola, a deputy who backed the bill, told reporters. "If we're all equal before the law, why do we want to give a different name to unions between same-sex couples?"

The dramatic vote, wrapped up at 2.25am after 12 hours of debate, ended with 125 in favour, 109 against and six abstentions. President Cristina Kirchner said she would not veto the bill, which would also allow gay couples to adopt children. The last obstacle to it becoming law is the upper house, which is due to vote this week. With senators free from party whips to vote according to their conscience, few dare predict the outcome.

The Catholic church branded the legislation immoral and perverse but there have been no big street protests like those that shook Spain during its debate on the issue. Argentina remains mainly Catholic but hostility to homosexuals has waned in the last decade, a trend seen across Latin America, according to Latinobarometer polls.

Many countries permit civil unions and some, like Uruguay, allow adoptions. Mexico City – though not the rest of Mexico – allows gay marriage. Argentina would be the first country in Latin America to grant homosexuals the same rights as heterosexuals. Buenos Aires is a gay-friendly capital with bars, restaurants and hotels catering to the pink pound. Five gay and lesbian couples recently married after local judges authorised the ceremonies, claiming the constitution supported freedom of choice for couples. Other judges overturned some of those marriages as illegal, leaving the issue in limbo. All eyes now turn to the senate.