President Emmanuel Macron called for calm on the eve of nationwide cost-of-living protests Saturday that authorities fear could lead to a violent escalation of weeks of student demonstrations.
With the rallies promising a fraught six month run-in to the country's presidential election, Paris authorities banned protests in the heart of the capital.
The country's Yellow Vest movement, which eight years ago staged street protests against Macron's government, called Saturday's "yellow and youth" demonstrations.
It has appealed to teachers and students, workers, pensioners and farmers to take part.
"We're expecting a pretty big turnout," Interior Minister Laurent Nunez told France 2 television. A police source told AFP that authorities estimated nearly 200,000 people could take part, with 40,000 in Paris.
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Macron called "for calm and respect". In Brussels for European Union talks, he added the right to protest went hand in hand with "respect for people and property".
Macron this week said he believed the "legitimate" demands of the students were being "exploited for political purposes", according to a government spokeswoman.
The unrest has been seized upon by France's far-left and far-right parties.
Jean-Luc Melenchon, leader of the far-left France Unbowed party, has backed the student protests and marched in one demonstration Tuesday supporting them.
Marine Le Pen, who leads in presidential election opinion polls, has positioned her National Rally party on the side of the police.
The prospect of the student protests merging with public anger over the cost-of-living could weigh heavily on Macron's government ahead of the two-round election in April and May.
Yellow Vest resurgence
"I have a feeling that (Saturday) will be a sort of culmination," 18-year-old Marie-Lou, a protesting university student in the city of Lille, told AFP.
"We'll be joined by workers and by the Yellow Vests, who are also mobilising again. There's a sense of hope that all these different struggles might converge."
Prime Minister Sebastien Lecornu has ordered a crisis cell to monitor the demonstrations. On Thursday he said he feared clashes after what he called "urban violence" during the student protests.
Lecornu is attempting to get parliament to pass a cost-cutting budget, while protesters are demanding greater protection from inflation and more spending on public services, particularly education.
Anger at police
There is also public anger at perceived police brutality during the student protests. Several teenage protesters have been seriously injured and thousands arrested.
"It's not just a Yellow Vest movement, it's not just a student movement, it's all the people who are rising up and that's why those in power are afraid," said Benjamin Ball, an activist working on a website for Saturday's demonstrations.
Organisers of Saturday's demonstrations have set up a website showing more than 600 demonstrations planned across France. Authorities and demonstrators were preparing for possible violence.
Paris police chief Patrice Faure banned demonstrations around the Elysee presidential palace, the prime minister's office, and the Champs-Elysees shopping avenue, among other key locations.
Police also banned a major march through the heart of the capital and negotiations with organisers over an alternative route failed to reach agreement. The main Paris demonstration was to kick off at 2:00 pm (1200 GMT) at the Republique square and head to northern Paris.
Nunez told France 2 that officers could use stun grenades in case of "extremely serious urban violence". He had suspended their use during the student protests after one stun grenade blew off the hand of a 15-year-old.
"Disturbances are expected, especially in the demonstrations in Paris and other big cities," an interior ministry official told AFP on condition of anonymity, citing a risk of violence from "the ultra-left movement".
"Actions blocking major roads or large, essential facilities will not be tolerated," the official said.
Demonstrators have set up their own smartphone app giving the real-time location of police, tear gas and helicopters.