Clutching French flags, dressed in pink and blue, the colours of the movement, carrying children or pushing buggies, protesters shouted slogans against President Francois Hollande as they made their way through the city.
"We've been to all the protests," said a 32-year-old mother who only gave her first name Camille, as she breast-fed her four-month-old son.
"We're here for children's rights. We don't want the state to be complicit in a child being deprived of a father or a mother," she said.
The mass protest was staged just two days ahead of a decisive parliamentary vote on the bill, which also allows adoption by gay couples. If passed it will make France the 14th country in the world to legalise same-sex marriage.
Organisers and security forces were on alert after violence marred anti-bill protests last week and amid fears that far-right extremists intent on sparking unrest had infiltrated the opposition movement.
The leader of the far-right "Nationalist Youths" group, Alexandre Gabriac, was among those marching. "We have around 50 nationalists in the protest," said Gabriac, who last week was detained after clashes with security forces.
"We want a peaceful demonstration and we reject all groups that directly target homosexual people," said the humorist known as Frigide Barjot, spokeswoman for the "Manif pour Tous" group driving the anti-bill protests.
Police sources said officers had detained three protesters carrying tear gas canisters.
Paris police estimated the march attracted 45,000 people, while organisers said 270,000 turned out on a sunny afternoon in the French capital and the march passed off peacefully.
Late on Sunday however, on the Esplanade des Invalides near parliament, police scuffled with a few hard-core protesters who had stayed on after the rally, and made some arrests. But there were no major clashes.
Opinion polls regularly show that while a slim majority backs same-sex marriage, a similarly narrow majority opposes adoption by gay couples.
The bill tackling both issues has proved hugely divisive in a country which, while officially secular, is still predominantly Catholic.
Tension over the imminent adoption of the bill reached breaking point last week.
In parliament's lower house, the National Assembly, a final debate on the legislation was marred by scuffles between deputies early Friday.
On the streets of Paris, opponents of the measures marched for three nights in a row and activists, some wearing masks, clashed with police, who made more than 100 arrests. Two journalists were attacked during one march and cars were vandalised.
Rights groups have also reported a rise in verbal and physical assaults against homosexuals, and two gay bars came under attack last week, in the northern city of Lille, and in Bordeaux in the southwest.
Elsewhere in the capital Sunday, supporters of the legislation gathered in Place de la Bastille, one of the city's main squares, to protest against homophobia -- around 3,500, said police. Organisers put the number at 15,000.
"Those who are for more equality must also make themselves heard," said the mayor of Paris, Bertrand Delanoe, who is himself gay. He denounced the recent violence as "a form of barbarism and regression".
Supporters of the bill, while less vocal than those against it, have also staged large-scale demos in recent months.
"We wanted to show that sexual minorities can be visible, that we are there and that our lives have the same value as those of others," said Laure Pora from the pro-gay marriage group Act-Up.
The bill is 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.
The UMP has accused the government of rushing the bill through its final legislative stages.