Tens of thousands of demonstrators crowded onto French streets on Saturday to denounce the rise of the country’s far-right political party and call on fellow citizens to block it from taking power in snap parliamentary elections set by President Emmanuel Macron.

The protests, organized by the country’s five biggest labor unions, were widely supported by human rights associations, activists, artists and backers of a newly formed left-wing coalition of political parties, the New Popular Front. Most protesters painted a dark picture of the country under a far-right prime minister.

“For the first time since the Vichy regime, the extreme right could prevail again in France,” Olivier Faure, the leader of the Socialist Party, said while addressing the crowd in Paris.

That prospect brought out of retirement former President François Hollande, who announced on Saturday that he would run for legislative elections to help ensure that the far right would not take power.

“The situation is very grave,” he said, in his hometown, Corrèze. “For those who feel lost, we need to convince them: The coming together of the French is indispensable.”

Mr. Macron shocked the country last week by announcing that he was dissolving the lower house of Parliament and calling for new parliamentary elections after his centrist Renaissance party was clobbered by the far-right National Rally party in elections for the European Parliament.

The move is a political gamble; Mr. Macron hopes voters will rally to him. He is painting himself as the clear force of sanity and stability between two extreme forces — the National Rally and the far-left France Unbowed party, which has since joined the New Popular Front.

But there are signs that his decision may backfire.

Early polls show a lead for the National Rally, which has long called for a drastic cut to immigration and asylum seekers and the introduction of a system of “national preference” that would reserve jobs, housing and hospital treatment for native French people.

“There were many things behind Macron’s gamble to call this election,” said Gilles Ivaldi, a political science professor at the Paris-based Sciences Po university who studies far-right politics in France and Europe. “One thing he missed — there is political momentum for the National Rally. That’s key to winning elections.”

The rapid upswing in the National Rally’s political fortunes, after years spent blocked from power, drew people like Philippe Noel, a 45-year-old teacher, into the streets on Saturday.

“There is a real risk that we end up with a far-right government” Mr. Noel said, as he walked by a brass band playing pop songs for the crowd under a drizzly sky. “But it’s not inevitable, and I hope the parties of the left can all unite.”

By Saturday afternoon, 250,000 people had come out across France, including 75,000 in Paris, according to estimates from the police.

“I came because I am angry and I feel powerless,” said Lucie Heurtebize, 26, who works in the technology industry. “We need to unite.”

As the protests began to wind down, Gabriel Attal, the prime minister and a member of Mr. Macron’s party, announced changes to Renaissance’s platform that mirrored pledges their rivals have put forward to increase spending power for French households. These included indexing pensions to inflation, allowing employers to increase employees’ salaries through untaxed bonuses and providing supplementary health care coverage at a cost of 1 euro per day.

“The French know that our program is coherent,” he said in an interview with Le Parisien, a daily newspaper. “With the other parties, it’s a parachute jump without the parachute.”

The demonstrations passed largely without incident, as people of all stripes — students, workers, tech programmers, business executives and soccer players — voiced their determination to stop the rise of the National Rally.

But in Paris, groups of protesters clad in black began smashing storefronts before clashing with throngs of riot police officers, who deployed tear gas. Those protesters drew boos from other demonstrators and warnings that the violence would help right-wing politicians and right-wing media outlets cast those on the left as extremists.

“It’s not normal that today we have 50 percent of people who vote for a racist, misogynist party that wants to expel foreigners and doesn’t address the real problems of the people,” said Laura Michaud, 31, a business executive who came to the protest with friends. “I’m not a fan of Emmanuel Macron, but if I have to, I will vote for him.”

Many in the crowd, however, said they hoped the newly-formed, left-wing coalition could beat out the far right, as the original Popular Front coalition had during the 1930s.

Coalescing against the rise of far-right fascist political groups in Europe, the original Popular Front formed a government under Léon Blum, who in 1936 became the country’s first Socialist and Jewish premier. Under his brief leadership, Mr. Blum secured many workers’ rights considered essential today, including collective bargaining, a 40-hour workweek and two weeks of annual paid vacation.

“We had been expecting this Popular Front for a long time,” said Patrick Franceschi, a business developer who supports the Greens party. “We voted for Macron twice to oppose the National Rally, but now there is a front of the left and it’s closer to my political family.”

The New Popular Front is made up of ecologists, communists, socialists and far-left parties, which came together despite recent bruising conflicts and opposing policies.

On Friday, the coalition unveiled a platform promising to increase wages, restore purchasing power to French citizens and lower France’s legal retirement age to 60 from 64. It said it would pull away from the European Union’s free trade treaties, which the coalition says kill French jobs through globalization.

On immigration, the coalition wants France to become more welcoming to asylum seekers and climate refugees — a position drastically different from the National Rally’s proposal.

But cracks have already appeared, with some prominent members of France Unbowed being pushed off the list of candidates. And the return of Mr. Hollande could add more complications; the Socialist Party crumbled under his leadership, and he is a polarizing figure for many on the left.

Saturday’s demonstrations echoed large-scale protests that flooded streets across France in 2002. At that time, the founder of the far-right National Front, Jean-Marie Le Pen, made it into the second round of the presidential elections. Mr. Le Pen’s daughter Marine took over the party in 2011, and changed its name to the National Rally, but its foundational ideas — opposing immigration and boosting the powers of the police — remained.

Back then, left-wing parties came together to form a so-called “Republican front” that asked members to protect the country against the far right and cast their votes for Jacques Chirac, the conservative competitor, even though they disagreed with his policies.

“It was tracing a line in the sand,” said Cécile Alduy, a professor at Stanford University and an expert on the National Rally, declaring that “there was an essential distinction between a party that threatens the Republic by breaking values like equality and freedom and solidarity, and other parties that you might disagree with on policies, but they fall within the framework of the Constitution.”

It worked. Mr. Chirac was overwhelmingly voted into office as president.

Since then, a Republican front-style bulwark has been called for repeatedly, particularly during lower-level elections, to keep far-right party members out of the president’s office. And while the strategy has worked in the past, it has gradually weakened. In 2022, 89 National Rally members were elected to the 577-seat National Assembly, making the party a formidable opposition force. Ms. Le Pen received 41.5 percent of the vote in the presidential election, though she lost to Mr. Macron.

Last week’s European elections saw the party make even more gains.

Ségolène Le Stradic contributed reporting.

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