Tens of thousands of people were expected to cram the streets of Paris Sunday in a last-ditch protest to stop controversial gay marriage legislation from being approved.
On Tuesday the French parliament is expected to pass the bill allowing gays to marry and adopt children, after weeks of opposition protests which culminated this week in several successive days of hostile, and sometimes violent, rallies.
Far-right former National Front leader Jean-Marie Le Pen said Saturday he was amazed by the "extraordinary" and "practically permanent" opposition to the reform.
"It is normal that people demonstrate against a bill they oppose, but it's a surprising phenomenon, including for people in power, to see that (the demonstrations) concern hundreds of thousands of people," he said. "It's quite exceptional."
A demonstration in March was attended by more than 300,000 people -- though it is unlikely that Sunday will see the same high turnout.
Supporters of the bill -- a key manifesto pledge by France's Socialist president Francois Hollande -- have planned a rival demonstration for Sunday.
The bill should see France become the 14th country in the world to legalise same-sex unions.
Tension over the imminent adoption of the law -- which is going through a second reading after already being approved in the French parliament's upper and lower houses -- reached breaking point this week.
Demonstrators marched for three nights running in Paris, with skirmishes with police on Friday resulting in officers using tear gas on protesters and making one arrest.
Nationwide protests a day earlier saw 75 people detained and, on Wednesday, two journalists were attacked and a number of cars along the rally route in the capital vandalised.
Meanwhile on Friday in parliament's lower house, the National Assembly, a final debate on the legislation was marred by unprecedented scuffles between deputies on opposite sides of the argument.
Opponents have accused the government of rushing the bill through its final legislative stages by implementing a fast-track measure that has limited debate to 25 hours.
NGOs have reported a general rise in verbal and physical assaults against homosexuals amid the debate, with two gay bars in the northern city of Lille and in Bordeaux coming under attack Wednesday.
Hollande, whose support for his campaign promise has never wavered, has condemned the surge in "homophobic" violence. Interior Minister Manuel Valls warned that far-right organisations are infiltrating the opposition movement, triggering unrest.
The bill was largely supported by the ruling Socialists, their allies in the Green Party and the Communists, and opposed by the main opposition UMP and other right-wing and centre-right parties.
It has proved hugely divisive in a country that is officially secular but predominantly Catholic.
Sunday's march through central Paris, due to start at 1230 GMT, hopes to gather "between 30,000 and 50,000 people" organisers said.
"We only have a few days left, we are not going to abandon the street" protests now, said organiser Alberic Dumont on Friday.
Delegations are expected from both the National Front and the UMP party -- which launched an official call to mobilise.
Opinion polls have routinely indicated that a majority of French people support gay marriage but that fewer support adoption rights for homosexual couples.
A further mass protest in Paris is planned for May 26 if the law is approved, to demand its withdrawal and a referendum on gay marriage.