People were to take to the streets for the fifth Saturday in a row, with a main rally in Paris from 1:00 pm (1200 GMT) and other gatherings across the country.
The Interior Ministry estimated "between 50 and 60 marches" nationwide, with between 30,000 and 60,000 demonstrators expected in Paris.
Some 70,000 police officers and gendarmes have been deployed.
France has been gripped by high-school blockades and cost-of-living demonstrations for more than a month, ahead of presidential elections next year that the far right sees as its best chance of seizing power.
Some of the largely peaceful rallies have seen fringe violence, as well as accusations of police brutality.
In the capital, the Yellow Vest movement would join high-school students, unions and other activists fighting for children's rights or opposed to a bill to give police more protection in their use of weapons.
"Society's in bad shape," said Claire Bourdille, from a collective battling violence against children and teenagers. "We can't focus on one cause more than another."
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Activists have in recent months demanded better protection for children against sexual abuse.
Others will be protesting against a new bill in parliament to grant police who use their weapons a presumption of legitimate self-defence.
Backers, including the far-right National Rally party of presidential hopeful Marine Le Pen, say it is necessary to avoid police being automatically suspected of using disproportionate force.
Activists argue the bill, approved by the lower house and soon to go to the Senate, amounts to a "licence to kill".
Activist fatigue
They say it would make holding security forces accountable even harder in a country where most cases of alleged police brutality rarely reach a criminal court.
Lawyers last week denounced alleged police violence at recent protests that led to "mutilations" including hand, foot and eye injuries.
Karl Ghazi, of the CGT union, said Saturday's protest in Paris might be different with "real activist fatigue" after a month and a half of protests.
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"It'll essentially be child rights activists on one side and people against the 'licence to kill' on the other," he said.
Around 146,000 people took part in demonstrations across France on Saturday last week, according to authorities, far lower numbers than on previous weekends.
The largest protests in recent weeks occurred during a union-led strike on Thursday last week, when more than half a million people took to the streets nationwide, according to official figures.
Unions have called for a new day of strikes on Tuesday, after rejecting a government promise to increase the minimum wage by 0.6 percent as "crumbs".
(with AFP)
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