The French parliament is
expected to approve a bill legalising same-sex marriage and adoption,
following a divisive public debate.Opponents of the move have staged some of the biggest protests seen in France in years, though polls show a small majority in favour of gay marriage.
Demonstrations are expected from both supporters and opponents of the bill.
Police reinforcements have been brought in and security stepped up. Although previous rallies have been overwhelmingly peaceful, some clashes on the fringes have been blamed on far-right elements.
President Francois Hollande has made the law his flagship social reform.
He is expected to add his signature to the bill once it has cleared any challenge in the constitutional council.
Opponents of gay marriage hope the council will block the bill - but analysts say that is unlikely.
The Senate has already approved the bill and the lower house, the National Assembly, is expected to do so on Tuesday.
Threats condemned
They also say the campaign of opposition has encouraged an increase in violent attacks on gay people.
There have been some high-profile attacks in recent weeks, and on Monday gunpowder was posted to the speaker of the lower house, demanding he stop the vote.
The violence and threats have been condemned by both sides.
Some argue this is the most important social reform in France since the death penalty was banned in 1981, says the BBC's Christian Fraser in Paris.
France will become the ninth
country in Europe to allow same-sex marriage. It has already been
legalised not just in the traditionally liberal Netherlands and
Scandinavia, but also in strongly Catholic Portugal and Spain.
Legislation is also moving through the UK Parliament.
But the measure has aroused stronger-than-expected opposition in France - a country where the Catholic Church was thought to have lost much of its influence over the public.
In January, a protest in Paris against the bill attracted some 340,000 people according to police - one of the biggest public demonstrations in France in decades. Organisers put the figure at 800,000.
Since then both sides have held regular street protests.
The opponents, backed by the Catholic Church and conservative opposition, say France already has civil partnerships for homosexual couples, and extending rights to marriage will undermine an essential building block of society.
Opinion polls suggest about 55-60% of the public support same-sex marriage.
However, support for adoption by same-sex couples, which is also provided for in the bill, is only at about 50%.
Opponents have marched under banners declaring a child's right to a mother and father.
At a last-ditch demonstration on Sunday, 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."