Earlier in the week, Catholic officials across Argentina, where more than 90 percent of the population are members of the church, read a message from Buenos Aires Archbishop Jorge Bergoglio urging the faithful to oppose the bill.
|Church groups have said the proposed law is a threat to traditional family life [AFP]
Outside Congress on Wednesday, gay rights supporters chanted abuse against the Catholic Church, equating it with "garbage... dictatorship," while anti-gay groups prayed and held up images of he Virgin Mary.
Police had to form a barrier between the two groups after some skirmishes broke out.
At the start of Wednesday's debate, ruling Justicialista Party Senator Miguel Angel Pichetto said 37 of the 72 lawmakers needed for quorum were present, with "between 33 and 35" in favour of the measure.
But he added, "let's see how (the vote) turns out in the end."
Cristina Kirchner, the Argentine president, has been pushing the gay marriage bill through Congress, even though it was originally sponsored by the opposition Socialist Party.
Kirchner has slammed the Catholic Church for leading what she called was "a holy war" against the measure.