Thousands of demonstrators opposed to same-sex marriage have gathered outside Argentina's congress ahead of a key vote by lawmakers, the Associated Press reports.

Supporters of the measure also took to the streets in loud rallies in the capital and other cities.

Argentina's House of Deputies has approved same-sex marriage and sent the legislation to the Senate for consideration on Wednesday. President Cristina Fernandez promises not to veto the measure if it reaches her desk.

The legislation would open the way to adoptions by same-sex couples and faces strong opposition from the Roman Catholic Church and other religious groups.

The main slogan for Tuesday's protest against the legislation was "the children have a right to a mother and a father."