Members of more than 100 left-leaning organisations participated in the “Liberty March” in cities and towns across the country.
The protests were the first opportunity for a divided left to take to the streets after a year and a half of Covid-19 restrictions.
Organisers reported 70,000 participants in Paris and 150,000 around the nation, while the Paris police and interior ministry put the numbers at 9,000 in the capital and 37,000 nationwide.
The interior ministry said 119 rallies had taken place.
In Nantes, western France, around 900 people rallied, according to the local prefecture, including scores of far-left militants who clashed with police.
In the Mediterranean port of Marseilles, more than a thousand demonstrators marched behind a CGT union banner that called for “unity to break down the capitalism that leads to fascism”.
Protesters vented against issues ranging from recent legislation they say chips away at liberties, such as a law that could see prosecutions for publishing images of police officers in action, to what they charge is a creep of far-right ideas into the mainstream ahead of next year’s elections.
In the southern city of Toulouse, a 54-year-old teacher and union activist who gave his name as Gauthier remarked that students had begun to challenge him and warned that “extreme right ideas are gaining ground”.
Far-right ideas “are no longer the monopoly of far-right parties and … have now largely penetrated the political class,” said Benoit Hamon, the Socialist presidential candidate in 2017.
In Paris, far-left leader Jean-Luc Melenchon had flour thrown in his face as he spoke to reporters.
A suspect arrested later in the day claimed to be a “sovereigntist” who social network specialists said broadcast far-right commentary on YouTube.
The move against Melenchon, who has been accused of fuelling conspiracy theories ahead of the presidential election, came days after President Emmanuel Macron was slapped in the face while shaking hands with people on a regional visit.
Other events that have caused concern in France recently are allegations of ties between far-left figures including Melenchon and Islamists, a YouTube video that simulated the execution of a militant from his France Unbowed party, and university gatherings at which Caucasian participants were allegedly not allowed to speak.
Jordan Bardella, vice president of the far-right National Rally (RN) party, dismissed the demonstrations on Saturday as a bid to deflect attention from Melenchon’s remarks on terrorism and the 2022 presidential election.
Groups that took part included Socialists, Communists, ecologists and trade unions.