At least 45,000 people have attended the last big rally against same-sex marriage and adoption in Paris before the bill is expected to become law.
Rejecting the police estimate of the turnout, organisers said 270,000 people had attended the march.
The event unfolded peacefully despite fears of clashes after incidents at previous protests this year.
At least 3,500 people joined a rival march in support of the bill, which is likely to be passed on Tuesday.
President Francois Hollande and his ruling Socialist Party have made the legislation their flagship social reform since being elected nearly a year ago.
Mr Hollande has been struggling with the lowest popularity ratings of any recent French president, with his promises of economic growth so far failing to bear fruit and unemployment now above 10%.
Opinion polls have suggested that around 55-60% of French people support gay marriage but only about 50% approve of gay adoption.
The anti-gay marriage lobby, backed by the Catholic Church and conservative opposition, argues the move will undermine an essential building block of society.
'We're not fascists'
The National Assembly is expected to pass the bill with no further debate on Tuesday afternoon, after it was adopted by the Senate earlier this month.
Opponents gathered on Denfert Rochereau Square on the Left Bank of the city under a banner which read: "All born of a Mum and a Dad".
"We warned the president back in November that we would not give up and that we would do everything to stop this law being passed, or to get it repealed if it is adopted," one of the protest organisers, Alberic Dumont, told Reuters news agency.
Numbers were smaller than at previous rallies, in part because no arrangements had been made this time to bring in demonstrators from the regions.
The protest's leader, the comedian known as Frigide Barjot, said care had been taken to prevent far-right militants joining the march.
Camille, 32, told AFP news agency as she fed her baby of four months: "We are here for the rights of the children... We take it as a slur when we are called homophobes or fascists."
Six people were arrested at the rally for carrying "dangerous materials" or for public order offences, but no major incidents were reported.
On Bastille Square on the city's Right Bank, the city's openly gay Socialist mayor, Bertrand Delanoe, headed the rival march, which organisers say attracted 15,000 people.
Mr Delanoe told AFP: "Those who support more equality must also be heard." Warning that "feelings of hate" were surfacing, he said people should not become resigned to them.
About a dozen countries have legalised gay marriage and gay adoption since the Netherlands led the way in 2001.