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.