Wal-Mart Stores has suspended one of its seafood suppliers in the South as an advocacy group for foreign workers pressed the retailer to improve working conditions there and at a dozen other suppliers cited for hundreds of federal labor violations.


The advocacy group, the National Guestworker Alliance, said on Friday that it had found terrible conditions at C. J.’s Seafood, a crawfish company in Breaux Bridge, La. Several immigrant workers said they had been forced to work 16 to 24 hours consecutively and had even been locked into the plant. Guest workers said they sometimes labored more than 80 hours a week, had been threatened with beatings to press them to work faster and had been warned that their families in Mexico would be hurt if they complained to government agencies.


“It’s one of the worst workplaces we ever encountered anywhere,” said Scott Nova, executive director of the Worker Rights Consortium, a university-sponsored monitoring group that was asked by the guest-worker advocates to investigate C. J.’s Seafood. “The extreme lengths of the shifts people were required to work, the employer’s brazenness in violating wage laws, the extent of the psychological abuse the workers faced and the threats of violence against their families — that combination made it one of the most egregious workplaces we’ve examined, whether here or overseas.”


Guest workers are temporary workers from abroad who typically receive special visas to do seasonal work.


On Friday, the National Guestworker Alliance released a list of 644 federal citations at 12 other Wal-Mart food suppliers that employ guest workers and used that list to assert that the retailer had fallen short on ensuring that its suppliers complied with its standards.


Lorenzo Lopez, a Wal-Mart spokesman, said the retailer had begun its own investigation of C. J.’s, which supplied its Sam’s Club warehouse stores, and had uncovered violations of some of its supplier standards.


“We have suspended C. J.’s Seafood as a supplier, pending the outcome of the investigation,” Mr. Lopez said.


He said that the United States Labor Department and the Occupational Safety and Health Administration were conducting their own investigations of the seafood processor.


C. J.’s did not respond to several phone messages left at its main office.


The Worker Rights Consortium’s investigation of C. J.’s found that some women worked from 2 a.m. to 6 p.m., that the workers were on average paid 42 percent less than legally required and that when the workers tried to complain to their managers, they were threatened with discharge, deportation and blacklisting. One worker told investigators that he was once forced to stay when he sought to leave after working 38 of the previous 48 hours.


After finding serious problems at C. J.’s, the Guestworker Alliance said, it decided to examine 18 other Wal-Mart food vendors that used guest workers.


The alliance said it had discovered the 644 citations at 12 of the companies. While many of them go back to the 1980s, there were 201 safety and other labor violations over the last five years, including 132 that OSHA had deemed serious.


“Wal-Mart is the nation’s biggest buyer and sets standards across the retail industry,” said Saket Soni, director of the Guestworker Alliance. “We want to have a productive conversation with Wal-Mart about its national supply chain standards.”


While acknowledging problems at C. J.’s, Mr. Lopez strongly objected to the Guestworker Alliance’s report and its suggestion that the citations at the other 12 suppliers indicated a possible problem with forced labor there.


“This report, crafted by a union-funded, union-backed group, has little to do with solving real issues,” he said. “It simply repackages old data from up to more than 20 years ago in an attempt to make people believe they have uncovered something new.”


“We work with more than 60,000 suppliers in the U.S., and we have rigorous standards in place that our suppliers are required to follow,” Mr. Lopez said.


He said that if the allegations of forced labor at C. J.’s are found to be true, it would permanently terminate its relationship with that company. The Guestworker Alliance’s report pointed to 33 citations at Tanimura & Antle, a vegetable grower based in Salinas, Calif. The report noted that 18 of the citations had incurred a total of $52,000 in fines since 2006, including one for failure to provide proper training after a worker suffered a severe eye injury when a tractor tire he was inflating exploded.


Rick Antle, the company’s president and chief executive, said it employed 2,200 workers and that 33 citations over two decades was “pretty good.” He said Tanimura & Antle was an “impeccable” employer and provided health and retirement benefits. He noted that none of the citations had involved guest workers.


The alliance’s report also focused on Trident Seafoods of Seattle, saying the company had received 212 OSHA citations and been fined $102,738.


More than half the citations were from 2000 or earlier. But in March 2011, Trident was fined $11,645 after OSHA found that its workers had been exposed to live electrical equipment carrying up to 480 volts, that a major electrical cable had exposed areas and that a high-pressure hose connected to a combustible gas container had not been properly tightened.


Trident said in a statement that any attempt to tie it to a labor abuse scandal “is patently false” and that “Trident’s plant safety and human resource policies are among the best in the seafood industry.”