France's ruling Socialists have refused to back down on laws to legalise gay marriage and adoption, a day after hundreds of thousands of people marched against the plans in Paris.

Despite the massive turn-out in Paris on Sunday - 800,000 according to organisers, 340,000 according to police - a statement from president Francois Hollande's Elysee Palace after the rally made it clear he would press ahead with the reforms.

The government plans to put the proposed legislation to parliament on January 29.

"This is a bill that strictly takes nothing away from anyone, which does not do away with the words father and mother...," justice minister Christiane Taubira told French television channel TF1.

Once again, she ruled out a referendum on the issue which she argued would in any case be unconstitutional.

But the head of the centre-right opposition UMP party's parliamentary group, Christian Jacob, insisted a referendum was perfectly feasible.

"The president of the republic can at any time decide to submit a question to referendum: I don't think there is any appeal against that," he said.

UMP leader Jean-Francois Cope meanwhile announced a special convention on the issue of the family for January 24, at which the issue of gay marriage would be discussed.

"This is an important test for Francois Hollande because you can see very clearly that there are millions of French people who are very concerned about this reform," he said.

Mr Cope was among those who took part in Sunday's protest, along with many other senior UMP figures.

Opponents of the bill had travelled from all over France for a demonstration supported by the centre-right opposition, the Catholic Church and France's five million strong Muslim community.

Neither the government's apparent determination nor the grim winter weather appeared to dampen the spirits of the protesters.

Giant marches converged near the Eiffel Tower having set off from three different starting points.

Many of the protesters were accompanied by children, some of whom brandished placards exclaiming: "Born of a man and a woman."

Despite months of well-attended protests however, opinion polls have shown consistently that most voters support the right of homosexual couples to marry, while a narrower majority favour granting them adoption rights.

Huge turnout: a demonstrator waves a flag on the Champ de Mars near the Eiffel Tower. ( Reuters: Charles Platiau )

AFP