After months of rancorous debate that has split France down the middle, proposals to allow gay couples to marry and adopt children finally go before parliament on Tuesday.

Recent weeks have seen hundreds of thousands of people taking to the streets to voice either opposition to or support for a reform that has been championed by Socialist President Francois Hollande.

With opinion polls having consistently shown that a comfortable majority of the French supported allowing same sex couples to marry, Hollande could never have anticipated that a promise he made in his election manifesto last year would generate such controversy.

Instead, a campaign orchestrated by the Catholic church and belatedly backed by the mainstream centre-right opposition steadily gathered momentum throughout the autumn and culminated in a giant protest in Paris two weeks ago.

Somewhere between 340,000 and 800,000 demonstrators flooded into the capital in a protest that was at least twice the size of a pro-gay marriage march staged on Sunday.

The debate has, at times, been bitter.

In September, Cardinal Philippe Barbarin, the archbishop of Lyon, claimed the government's plans to redefine the concept of marriage would open the door to incest and polygamy.

That prompted Bertrand Delanoe, the mayor of Paris and one of France's few openly gay politicians, to say the elderly cleric must have "flipped his lid."

Similar withering criticism was directed at Serge Dassault, the prominent industrialist, who suggested the French would die out after being consumed by the same decadence that led to the fall of ancient Greece.

"We'll have a land of homos," Dassault claimed. "And then in 10 years there will be no-one left. It's stupid."

The movement in support of gay marriage has been less strident but did produce one of the most iconic images of recent times, AFP photographer Gerard Julien's snap of a lesbian couple kissing in front of opponents of the planned legislation.

Throughout all the turmoil, Hollande's support for the legislation has not wavered and his girlfriend, Valerie Trierweiller, has revealed that the president will be attending the marriages of gay friends once the legislation is on the statute books.

That is expected to happen by the middle of this year with the Socialists enjoying an outright majority in parliament and the proposed reform also supported by the Greens, Communists and some centrists.

Parliamentary opponents of the legislation have tabled some 5,000 amendments but the guerilla tactics are thought unlikely to significantly delay or dilute the legislation.