The rally, held in front of the city hall under steady rain and watchful police supervision, brandished slogans decrying "hate-filled" rhetoric seen as encouraging violence against gays and lesbians.
"It's very important for us all to be here... to show an image of love in dignity," said one of the demonstrators, Wilfried de Bruijin, a Dutch gay man living in France who was brutally attacked in Paris last weekend while walking with his boyfriend.
Photos of de Bruijin's battered face have been shared on social networking sites thousands of times.
The protest came after France's Senate earlier Wednesday passed the first article of a bill that clears the way for a law enshrining the right for homosexual couples to marry.
The issue has stirred vehement debate in France, an officially secular but predominantly Catholic nation that has a long history of tolerance towards many varieties of sexuality as long as they are kept discreet.
In January, hundreds of thousands of demonstrators flooded into Paris for an anti-gay marriage march. Last month, police fired tear gas on people protesting the bill, and dozens were arrested.
Opponents said Wednesday they would organise another mass protest in Paris on May 26 if the law is approved, to demand its withdrawal and a referendum on gay marriage.
Polls in France regularly show that a majority of people support gay marriage, but far less are in favour of same-sex adoption.