Union bosses in this region of rural Mississippi have long grumbled that the largest factories here hire illegal immigrants, and that the immigrants were starting to get more overtime and supervisory positions.

Friction between the union and immigrant workers, along with a tipoff at an electrical manufacturing plant, boiled over this week into the biggest workplace immigration raid in the nation’s history.

When the first of the 595 suspected illegal immigrants was taken into custody Monday, some fellow workers broke into applause. A spokeswoman for U.S. Immigration and Customs Enforcement said the investigation started three years ago after agents received a tip from a union member.

In interviews with The Associated Press, both union members and immigrants spoke of a simmering tension. At least one immigrant said scare tactics were used to pressure people to join the union.

Union members said they resented immigrants, who were often allowed to work as much as 40 hours of overtime a week when other workers were discouraged from doing so. All declined to give their names, saying they feared for their jobs.

Howard Industries, which makes dozens of products from electrical transformers to medical supplies, is in Mississippi’s Pine Belt region, an area known for commercial timber and chicken-processing plants.

Robert Shaffer, head of the Mississippi AFL-CIO, said Wednesday that members have long complained that companies in southern Mississippi hire illegal immigrants.

“Jackson, Hattiesburg, Laurel and all areas along the coast, it’s a little Mexico,” Shaffer said. “I’m not against people trying to make living. I have a compassion for those folks. But at the same time, the taxpayers of Mississippi shouldn’t be subsidizing a plant that won’t even hire their own workers.”

In 2002, Mississippi lawmakers approved a $31.5 million, taxpayer-backed incentive plan for Howard Industries to expand. The company, with 4,000 workers, is the largest employer in Jones County, which includes Laurel.

About 2,600 of Howard Industries’ workers are in the union. Shaffer said he did not know whether any of those picked up in the raid were union members, or if nonunion workers were offered overtime while union workers were not.

Shaffer said offering immigrant workers union membership would depend on the situation, but he doubted it could be done if immigrants were in the country illegally.

Those detained in the raid came from Brazil, El Salvador, Germany, Guatemala, Honduras, Mexico, Panama and Peru.

Contacted Wednesday, Howard Industries referred reporters to the statement it issued Monday, which said the company “runs every check allowed to ascertain the immigration status of all applicants for its jobs. It is company policy that it hires only U.S. citizens and legal immigrants.”

No executives were detained in Monday’s raid, but a spokeswoman said the raid was just the first part of an ongoing investigation.

A 30-year-old immigrant from Mexico who has worked at the transformer plant for three years said union representatives pressured immigrants to join the union, sometimes visiting their homes, offering gifts such as shirts and indicating that if they joined the union they would make more money.