GRENOBLE, France — The striking workers had no battle plan, but their jobs were endangered by layoffs, and they were itching for a confrontation.

So when managers at the U.S.-owned Caterpillar factory here refused to negotiate under pressure, workers recalled, resentments that had built up during several years of increasingly sour labor relations suddenly boiled over. About 40 employees invaded the executive suite, locked five top bosses inside and said they would be released only after resuming talks on the strikers’ demands.

“It was spontaneous. We just had it with them,” said Benoit Nicolas, 38, a Caterpillar line supervisor and delegate from one of several striking unions, the General Labor Confederation. “They refused to talk, so we locked them up until they agreed to negotiate.”

The takeover, at midday March 31 in a Grenoble suburb in the Alpine foothills 75 miles southeast of Lyon, ended without injuries 24 hours later. It was one of more than half a dozen “boss-nappings” over the past month in factories across France, a whiff of revolution by workers who are facing massive layoffs because of the global economic crisis.

The hostage-takings, a specifically French reaction to the worldwide crisis, have been denounced as illegal by President Nicolas Sarkozy. But they have been widely applauded among the French people — and in some instances have brought results. Most of all, they have dramatized the extent to which, in France perhaps more than anywhere else, the perspective of class struggle remains lodged in many people’s minds and shapes the way they view the economic crisis.

The latest detention took place Thursday, when workers facing layoffs at a printer plant near Strasbourg run by Faure et Machet, a Hewlett-Packard contractor, confined their bosses in a meeting room for about 12 hours and forced them to continue negotiating on a severance package. Previously, a 3M executive in Pithiviers was held overnight after announcing layoffs, as were the head of Sony France in Pontoux-sur-Ardour and three expatriate British bosses in a Scapa Group adhesive tape plant at Bellegarde-sur-Valserine.

Trying another tactic, workers facing layoffs at the Celanese-owned Acetex-Chimie plant in Pau started a rotating hunger strike, with Mayor Martine Lignières-Cassou taking a turn to show solidarity.

More spectacularly, François-Henri Pinault, a luxury-brand magnate who recently married actress Selma Hayek, was surrounded in a car in the middle of Paris by salesclerks upset at layoffs in his stores. Before police came to his rescue, television cameras captured the Gucci millionaire negotiating through the car window and snapping to his captors that their actions were altogether inappropriate.

The historic class-struggle reflex has been sharpened because, in the view of many French workers, the current crisis is the fault of rapacious Wall Street speculators and their French equivalents. Reports of fat bonuses and stock options, even in businesses that accepted anti-crisis subsidies, have exacerbated the popular outrage. In opinion polls, about half of those queried support the workers who carry out boss-nappings.

“There is no justice in France today, and even less in America, because the bosses went to the casino with our pension money,” complained Rene Mirisola, 46, a machinist and 24-year Caterpillar veteran whose schedule has been reduced to part time.

Jérôme Pélisse, a sociologist at the University of Reims who specializes in labor conflicts, said the French labor union movement descended from a tradition of confrontation. The main Caterpillar plant here, for instance, sits next to Leon Blum Avenue, named for the socialist prime minister whose Popular Front government in the 1930s instituted paid vacations and the 40-hour workweek despite bitter opposition from French industrialists.

Moreover, Pélisse said, France’s perennially high unemployment rate means laid-off workers will have difficulty finding new jobs. “When they leave, they have to leave with a lot,” he said, drawing attention to the demand for severance packages that are at the core of many labor-management disputes, including Caterpillar’s.

Capturing the mood in its own way, the satirical newspaper Le Canard enchainé published a front-page cartoon Wednesday showing executives in a fancy restaurant, sharing tables with their mistresses and calling their wives on cellphones to explain that they would not be home for dinner because they had been abducted by workers. Another cartoon showed a bedraggled manager exiting his factory with strikers in the background and telling a companion, “I think I deserve a kidnapping bonus.”

In that vein, Pierre Piccarreta, another delegate from the General Labor Confederation, said union negotiators have demanded that executives at Caterpillar’s Grenoble factory give up their bonuses for 2008 and turn in their company cars to sweeten the pot for severance payments. The demand was rejected out of hand, he added.

“We live in a crazy world, and if things don’t change, there is going to be a revolution,” Piccarreta told a rally Tuesday, generating whoops from the workers. “We demand redistribution of the wealth that has been generated by Caterpillar. Today the workers are calling the tune.”

Workers at the rally, numbering about 600, shouted and whistled when Piccarreta urged more defiance. They booed and sent up catcalls when he brought up Sarkozy’s condemnation of their tactics. The refusal of Caterpillar executives to forgo company cars elicited cries of “scandal” and “shame.”

“They say we’re a bunch of hotheads,” screamed a bearded worker, seizing a microphone and addressing his colleagues. “We’re not hotheads. The problem is that they are jerks.”

Union organizers said they were convinced that the hostage-taking was a success, forcing management to take them seriously. Only after the executives in Grenoble were held captive during the night, the organizers said, did they call U.S. headquarters, get authorization to pay workers for several strike days and resume negotiations.

The chief executive of Caterpillar in Grenoble, Nicolas Polutnik, told employees in February that the factory would lay off 733 of its approximately 2,500 employees, citing a drastic decline in orders for Caterpillar’s earth-moving and construction equipment. Many workers already had been put on part-time schedules. Reducing the number of layoffs is the unions’ main goal, organizers said, but they are also seeking increases in the severance package.

Polutnik was one of the five senior executives taken hostage. The human resources director, Maurice Petit, was released at the end of the day because of a heart problem. But Polutnik and the other three spent the night in the executive suite with a rotating team of workers acting as jailers.

Since then, talks have been held in government offices with involvement by the central government’s regional representative and telephone discussions between the representative and senior Caterpillar executives in the United States, union officials said. The Grenoble management team now moves about town with bodyguards, they added.

Despite Sarkozy’s call for respect of law, French authorities have brought no charges against the Caterpillar workers or others who have taken executives hostage. But a Grenoble court on Friday ordered the removal of 19 workers who were camping at the factory entrance and harassing employees who tried to enter.