The French parliament is expected to pass the bill allowing gays to marry and adopt children on Tuesday, after weeks of opposition protests which last week culminated in several days of hostile and sometimes violent rallies.
More than 300,000 people took part in a demonstration in March , but it is unlikely that Sunday will see the same numbers.
Supporters of the bill – a key manifesto pledge by France's Socialist president Francois Hollande – have planned a rival demonstration, also on Sunday.
The bill should see France become the 14th country in the world to legalise same-sex unions following New Zealand, which passed its law last week.
Tension over the imminent adoption of the law – which is going through a second reading after already being approved in the French parliament's upper and lower houses – reached breaking point last week.
In Paris, opponents of the bill marched for three nights running Wednesday through Friday.
A hard core of activists, some wearing masks, clashed with police, who made more than 100 arrests during the week. Two journalists were attacked during Wednesday's march, and cars along the route of the rally were vandalised.
In parliament's lower house, the National Assembly, a final debate on the legislation in the small hours of Friday was marred by unprecedented scuffles between deputies.
Opponents have accused the government of rushing the bill through its final legislative stages by implementing a fast-track measure limiting debate to 25 hours.
Rights groups have reported a general rise in verbal and physical assaults against homosexuals amid the debate, and two gay bars came under attack on Wednesday, in the northern city of Lille and in southwestern Bordeaux.
On Saturday, several hundred people marched in the northwestern city of Nantes to denounce what they said was a climate of fear created by a "fascist" wave of homophobia.
Michele Gressus, vice president of the city's socialist group, denounced what she called "the words, deeds and gestures of fascists" in the recent protests against the bill.
In Bordeaux meanwhile, several hundred people marched Saturday to protest the proposed law, blowing whistles, sounding horns and striking saucepans.
Hollande, whose support for his campaign promise has never wavered, has condemned the surge in "homophobic" violence. Interior Minister Manuel Valls warned that far-right organisations are infiltrating the opposition movement, triggering unrest.
The bill was largely supported by the ruling Socialists, their allies in the Green Party and the Communists, and opposed by the main opposition UMP and other right-wing and centre-right parties.
It has proved hugely divisive in a country that is officially secular but predominantly Catholic.
Organisers say they hope Sunday's march through central Paris, due to start at 2:30 pm (1230 GMT), will gather between 30,000 and 50,000 people.
"We only have a few days left, we are not going to abandon the street" protests now, organiser Alberic Dumont said Friday.
Opinion polls have repeatedly indicated that a majority of French people support gay marriage but that fewer support adoption rights for homosexual couples.
A further mass protest in Paris is planned for May 26th if the law is approved, to demand its repeal and a referendum on gay marriage.