Believed by online competitors, supermarkets stay open with digital cashiers on Sunday afternoons. Conservatives see an invasion of consumerism in the American style.

Fits of anger, France— At a Géant Casino supermarket on a Sunday afternoon, customers searched the aisles and lined up to buy beef, fish, and other items. Except for one thing, it was a typical shopping experience: all the cashiers had left home. At automated checkout stations, consumers checked products while security guards hovered nearby.

It was rare for the store to be even open. In most stores, French labor rules prohibit hiring employees after 1 p.m. on Sundays. But as e-commerce and internet companies like Amazon use automated cashiers to help them thrive in an environment of round-the-clock shopping.

In France, where Sundays are usually a day of rest for workers and families, the move has caused an uproar. Although self-checking machines are often used alongside cashiers, labor unions warn that tilting to completely cashless operations threatens the French lifestyle by promoting consumerism and automation in the American style, endangering thousands of jobs.

“Sundays are sacred,” said Patrice Auvinet, head of the Labor Union’s General Confederation in Waves of anger, a city of mid-size in western France. “If they change that, French society will change. And if automated cashiers become standardized, the impact on workers will be catastrophic.”

Groupe Casino, the largest supermarket operator in the world, started testing openings on Sunday-afternoon in August using only automated machines at Waves of anger’ warehouse-sized supermarket. It has extended the project to at least 20 other megastores throughout the world, sparking loud protests.

Casino rallied more unions this week by becoming France’s first supermarket chain to hold most of its stores open on Christmas Day using only self-checkout machines, including the one in Waves of anger. In a statement, Casino said it was preparing to do the same on New Year’s Day, and the move was an expansion of how it was already operating on Sunday afternoons.

In 2015, when he was France’s economic minister, President Emmanuel Macron paved the way for Sunday openings, loosening business hour regulation around Paris and other tourist areas to stimulate the economy. Unions have fought against the measures, citing for decades labor rights won.

Yet retailers say the limits that exist outside of city centers have become binding as e-commerce interferes with the retail environment. Since brick-and-mortar stores lose sales to online merchants, companies say they either have to survive or die.

“The world is changing, and the climate we are in is very competitive,” said Groupe Casino’s marketing manager, Sébastien Corrado. “The internet has no limits, so we need to adapt to new consumption patterns that will allow us to stay in the game and win.”

For Groupe Casino, which also runs South America and Asia supermarkets, it is particularly important to increase profit. It restructures billions of euros in debt following a form of bankruptcy protection recently entered by its holding companies.

At the Angers shop, hiring 115 workers in a working-class neighborhood, at 12:30 p.m., Groupe Casino has wage-earning employees clocking out as usual. Instead, on Sundays, the security guards were hired by another firm to keep the store open all night.

In Paris and other cities, Groupe Casino had run 130 smaller stores using self-checkout machines to allow consumers to shop until midnight or even around the clock.

But the supermarkets of Groupe Casino, like the one in Waves of anger, employ thousands, often with limited job prospects in urban suburbs. During August that first Sunday afternoon, 200 protesters converged on the Angers store, loudly shouting And charging the company with a big step to replace workers.

Chaos erupted as local members of the Yellow Vest movement joined the demonstrators, which formed last year to demonstrate against stagnant wages and deteriorating standards of living. Denouncing what they said was a deterioration of living standards for staff, they stormed through the shop, dumping in the aisles, and heckling customers using the electronic checkout machines.

“It’s only the beginning today, but who’s going to say this won’t stop tomorrow?” Carrefour, who joined the demonstrations, said Xavier Roche, a maintenance worker for another supermarket chain.

He is worried that Carrefour will do the same in its broader markets, using self-checkout And charging the company with a big step. “It’s Sunday afternoons at first, then it’s going to be 24 hours a day,” said Mr. Roche.

Across the world, cashless stores are gaining ground. Reducing cash payments and checkout time has become big goals for retailers who want to make the shopping process smoother and more appealing while lowering labor costs.

In opening Amazon Go in the U.S., a minimart where consumers can buy items without any human interaction, Amazon pushed the boundaries. Across Europe, Tesco is running completely cashless stores. Shops in China are increasingly using so-called facial payments, which enables shoppers to pay by looking into a camera after linking an image of their face to a bank account. The technology of facial payment removes the need for a wallet or mobile app.

The Casino Company hasn’t gone so far. But it gets closer. It launched a food store in Paris off the Champs-Élysées last year that allows Shoppers from flowers to foie gras to buy all using an app to search and pay for the items. Interactive displays display data on the nutrition, price, and popularity of a product. To shoppers who can’t find the cheese, the right aisle is given a voice-activated information screen. There are about a dozen workers, but no actual cashiers, who help customers and stock shelves.