In Paris 50 arrests have already been made in anti-gay marriage and adoption law protests, which police say could draw some 200,000 people on Sunday.
The bill, supported by President Francois Hollande, passed into law May 18, but mostly right-wing protesters, including members of former president Nicolas Sarkozy's Union for a Popular Movement party (UMP), insist the fight is not over.
If the past is any indictor, the protests will likely be peaceful. But police expect a few "ultras" to mix with the crowds, and authorities think they might try and cause trouble.
France became the 14th country to legalize same-sex marriage, despite mass protests, especially in Paris, with one drawing about 470,000 people. Today, rally organizers hope more will come and make their "indignation" heard and "free France from the thought police."
Jacques Myard, a UMP member of parliament, told the BBC the dominant Socialist Party had passed the law "by force" and struck at "the French nation's conscience."
"Those people are playing God," Myard added, "because they want to marry, but it won't stop at this stage. Then they will adopt, and then we will have children in families where there is no father or no mother."
Opponents of the same-sex marriage law, such as Frigide Barjot, head of the anti-gay marriage movement La Manif Pour Tous, have suggested there could be efforts after the next local elections in 2014 to reverse the law.
On Tuesday, far-right writer Dominique Venner committed suicide in front of the Notre Dame Cathedral, but not before denouncing gay marriage and immigration. An essay on his website called legalizing gay marriage and adoption a "vile law."
La Manif Pour Tous tweeted this picture earlier today.