Unions are calling for the previous government's fiscal plans to be scrapped, for more spending on public services, higher taxes on the wealthy, and for the reversal of an unpopular change making people work longer to get a pension.
In Paris, many metro lines were set to be suspended for most of the day except for morning and afternoon rush hour. Pupils gathered to block the entrances to some schools.
"Block your high school against austerity," read a placard raised by a student in front of the Lycee Maurice Ravel high school in the French capital, where the gathering included teachers and workers' representatives.
"Workers are currently so despised by this government and by (President Emmanuel) Macron that, in fact, it can't continue like this," bus driver and CGT union representative Fred said at a rally in front of the high school.
Macron and his newly appointed Prime Minister Sebastien Lecornu are under pressure from parliament over the likely budget cuts and from investors concerned about the deficit in the euro zone's second largest economy.
An Interior Ministry source said as many as 800,000 people were expected to take part in the strikes and protests.
"The workers we represent are angry," the country's main unions said in a joint statement in which they rejected the previous government's "brutal" and "unfair" fiscal plans.
France's budget deficit last year was close to double the EU's three per cent ceiling but much as he wants to reduce that, Lecornu - reliant on other parties to push through legislation - will face a battle to gather parliamentary support for a budget for 2026.
Lecornu's predecessor, Francois Bayrou, was ousted by parliament last week over his plan for a 44 billion euro ($A78 billion) budget squeeze. The new prime minister has not yet said what he will do with Bayrou's plans, but he has signalled a willingness to compromise.
One in three primary school teachers were on strike nationwide, and nearly one in two walked off the job in Paris, the FSU-SNUipp union said.
Regional trains were heavily affected by the strike, while most of the country's high-speed TGV train lines worked, officials said. Protesters gathered to slow down traffic on a highway near the southeastern city of Toulon.
Interior Minister Bruno Retailleau told reporters early on Thursday that police had already removed some blockades, including in front of bus depots in the Paris region.
Some 80,000 police and gendarmes will be deployed throughout the day. Riot units, drones and armoured vehicles will be on hand.
