The French parliament has approved a bill legalising same-sex marriage and adoption, following a divisive public debate.
France becomes the 14th country to pass a law allowing gay marriage, and follows New Zealand last week.
Opponents of the move have staged some of the biggest protests seen in France in years, though polls suggest 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.
Supporters say gay people will only have equal rights when they are allowed to marry.
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.