After weeks of student protests, France faces broader, nationwide demonstrations Saturday over the cost of living, setting up a potentially fraught six months heading into a presidential election.
President Emmanuel Macron appealed for "calm" on the eve of the protests that authorities fear could see the two movements merge, perhaps with violence.
The country's Yellow Vest movement, which eight years ago staged street protests against Macron and his government, called Saturday's "yellow and youth" demonstrations.
It has appealed to teachers and students, workers, pensioners and farmers to take part in an online call spread through social media.
"We're expecting a pretty big turnout," Interior Minister Laurent Nunez told France 2 television.
Macron called on Friday "for calm and respect," adding the right to protest went hand in hand with "respect for people and property".
A police source told AFP that authorities estimated nearly 200,000 people could take part, with 40,000 in Paris.
"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.
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.
The unrest is weighing on the final months of Macron's presidency, ahead of elections in April and May to choose his successor.
It has been seized upon by opposition parties and their candidates.
Jean-Luc Melenchon, leader of the far-left France Unbowed party, has backed the student protests and marched in a demonstration Tuesday supporting them.
Far-right candidate Marine Le Pen, the election frontrunner according to surveys, has positioned her National Rally party on the side of the police.
Macron this week said he believed the "legitimate" demands of the students were being "exploited for political purposes", according to a government spokeswoman.
Organisers of Saturday's demonstrations have set up a website showing more than 600 demonstrations planned across France, and hundreds more protest "actions" such as blocking motorway toll booths or service stations.
Authorities and demonstrators were preparing for possible violence.
Paris police banned a major march through the tourist and business heart of the capital, and are locked in negotiations with organisers over an alternative route. The main Paris demonstration was to kick off at 2:00 pm (1200 GMT) at the Bastille square.
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.
Nunez told France 2 that officers could use stun grenades in case of "extremely serious urban violence". He had previously 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.