Big anti-austerity street marches and labor strikes gripped France on Thursday, raising the pressure faced by Sébastien Lecornu, the country’s new prime minister, as he tries to pass a debt-reducing budget by the end of the year.

Railway workers, students, civil servants and others joined protests organized by labor unions to oppose the plans of François Bayrou, Mr. Lecornu’s predecessor, to cut 44 billion euros, nearly $52 billion, from next year’s state budget. France’s Interior Ministry said over 500,000 people had attended the protests; one of the leading labor unions put the figure at over one million.

Mr. Bayrou was ousted by lawmakers last week, and President Emmanuel Macron replaced him with Mr. Lecornu, a centrist and an ally.

But it was not immediately clear whether Mr. Lecornu, who had promised a “break” with the past, would scrap his predecessor’s plans or use them as a basis for lawmakers to amend, and the concern over looming budget cuts has persisted.

“The budget that was imposed on us by the last prime minister — and the new one will be the same — was one of austerity,” said Cécilia Rapine, an archaeologist from Normandy who was hoisting an orange union flag at the march in Paris as protesters waved smoke bombs and danced to music blaring from trucks.

Ms. Rapine said she understood that France’s precarious finances needed fixing. But she worried that schools and hospitals would bear the brunt after years of business-friendly tax breaks that she said had deprived the state of funding.

“Austerity gives no hope,” Ms. Rapine said, as orange smoke wafted over her head. “It creates more cracks in society.”

Labor unions say that anything resembling Mr. Bayrou’s budget — which, among other measures, proposed to freeze welfare payments at their current level — would place an unacceptable burden on lower- and middle-class workers. The unions want higher taxes on wealthy individuals and big business, more funding of public services and a reversal of Mr. Macron’s increase in the legal retirement age.

“Today we are sending the government a very clear warning,” Marylise Léon, leader of the French Democratic Confederation of Labor, the country’s largest union, told reporters at the march in Paris. “The budget cannot be built solely on the back of workers.”

In Paris, many metro lines were running only during rush hour. Traffic was disrupted on regional train lines but mostly normal on the country’s high-speed rail route. Unions estimated that about a third of elementary-school teachers and 45 percent of middle and high school teachers were striking, though the Education Ministry cited lower figures.

Parts of the Louvre Museum were closed, and the Eiffel Tower shut down. The authorities even postponed a move of the Bayeux Tapestry, a medieval embroidery depicting the Norman Conquest of England, from a museum in Normandy to a storage area before a loan to Britain next year.

The marches were mostly peaceful but were marred in some cities by clashes between small groups of protesters, who threw projectiles or vandalized businesses, and riot police officers, who responded with volleys of tear gas. The Interior Ministry, which had deployed 80,000 security officers across the country on Thursday, said that the police had apprehended over 300 people in connection with the protests.

Nearly 200,000 people gathered around France last week for similar protests, initiated by a nebulous, uncoordinated online movement called Bloquons Tout, or Let’s Block Everything. While those demonstrations did not bring the country to a standstill, they attracted about twice as many people as the authorities had expected, signaling deep-seated discontent.

Mr. Lecornu quickly dropped one of his predecessor’s most unpopular ideas — scrapping two national holidays. He also announced that the government would curb certain perks for former cabinet members, like receiving police protection or getting a personal secretary and a driver. The move was mostly symbolic, as it is expected to save only a drop in the bucket compared with France’s deficit, which has ballooned to €166.6 billion, the largest since World War II.

“We cannot ask the French people to make efforts if those at the head of the state do not do so themselves,” Mr. Lecornu said last week in an interview with French newspapers.

But Mr. Lecornu’s first moves have not placated the social unrest so far. Mathieu Bertier, a prison worker marching in Paris with member of his union, said more was needed. If the government is “stubborn,” he warned, “it will be thrown out, too.”

To avoid the fate of his last two predecessors — both of whom were toppled by lawmakers — Mr. Lecornu has been consulting with political parties and labor representatives over the past week to hammer out his 2026 budget. In a statement after the protests, Mr. Lecornu said that he was committed to “continuing dialogue with all social partners” and that the protesters’ demands where “at the heart” of the consultations.

But the lower house of Parliament is divided into three blocs — left, center and far right — that disagree on how to address France’s finances, giving him a very narrow path to success.

To break the political deadlock, the nationalist, anti-immigrant National Rally party is pushing Mr. Macron to call parliamentary elections. The far-left France Unbowed party wants him to step down. But the president has rejected both demands and has asked instead that Mr. Lecornu find a budget deal.

That has put the more moderate Socialist Party in a key position. It has 66 lawmakers in the 577-seat lower house, enough to make or break a no-confidence vote. But its demands — such as a 2 percent tax on wealth above €100 million — run counter to Mr. Macron’s pro-business agenda.

After a meeting on Wednesday, Olivier Faure, leader of the Socialist Party, said that Mr. Lecornu had been evasive about his budget plans. “So far, we are left hungry for more,” he said.

Fitch Ratings downgraded France’s sovereign debt rating last week, citing the “increased fragmentation and polarization” of the country’s politics. The deficit is well above the 3 percent limit required in the eurozone. The national debt hit €3.35 trillion this year, or 116 percent of economic output, one of the heaviest burdens in the eurozone.

The instability has had more than just financial consequences.

Sophie Gal, a middle-school English teacher from a town near Montpellier, noted at the march in Paris that a series of cabinet shuffles and collapses over the past two years had produced a dizzying succession of education ministers.

“Every time, we need to restart,” she said. “Problems are never addressed.”