Paris police have stepped up security for the city's gay community ahead of a final parliamentary vote Tuesday on a bill that will make France the 14th country to legalise same-sex marriage.

After months of acrimonious debate and hundreds of street protests that have occasionally spilled over into violence, a reform that has split the country was expected to be comfortably approved by the Socialist-dominated National Assembly around 1500 GMT.

Deputies voted 329-229 in favour of the bill on its first reading in February and a similar outcome is expected in the ballot on the second and final reading.

Although the protests against gay marriage, some of them attended by hundreds of thousands, have generally been peaceful, the debate has taken on a nastier edge in recent weeks.

Some politicians have received personal threats, a handful of demonstrations have ended in violence amid claims of infiltration by extreme-right activists, and there was even a scuffle in parliament as the debate concluded in the wee small hours of Friday.

These tensions have been linked to a spike in hate crimes against the gay community that have included attacks on bars and two serious assaults in Paris, prompting the police to take preventive measures in case of a further backlash.

Bernard Boucault, the city's prefect of police, said the assaults, which took place on the night of April 6-7, had been almost certainly the result of homophobia.

"Everything possible is being done to identify those responsible and bring them to justice," Boucault said. "In order to ensure there is no repeat, we are reinforcing our presence in certain areas of the city at certain times."

Gay rights activists are planning a celebratory rally to coincide with the parliamentary vote and opponents will stage protests in Paris and across the country.

That will not however be the final chapter in a debate that has exposed profound fault lines in French society.

The bill, which will also accord gay and lesbian couples the right to adopt children, will only become law when it is signed by President Francois Hollande and published in the Official Journal.

Opposition parties are hoping to delay that step by challenging the measure through France's constitutional council, but the government is confident that will be dismissed.

"We have ensured that there is no legal weakness," said Family Minister Dominique Bertinotti. "The constitutional council is sovereign but the government is serene. We're confident."

Former president Jacques Chirac shelved an unpopular employment law that had been passed by parliament in 2006, but Hollande is seen as unlikely to emulate that precedent.

The president has stressed his personal commitment to the bill, despite struggling with some of the worst approval ratings any French president has ever endured.

The opposition Uf the economy and a recent scandal in which Hollande's budget minister was exposed as a serial liar and tax dodger.

The Socialist leader could scarcely have anticipated the scale of the opposition he would face over a reform that initially seemed to enjoy solid majority backing among French voters.

Recent polls have suggested a campaign in which the Catholic Church initially played the leading role has shifted opinion to the extent that the electorate is now fairly evenly split on both gay marriage and adoption.

Against that background, gay activists fear that Hollande will backtrack on a manifesto promise to give lesbians the same rights to IVF treatment as heterosexual couples who are unable to conceive naturally.