LAUREL, Miss.—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 spokesman 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 such industries as 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 a living. I have a compassion for those folks.

“But at the same time,” Shaffer added, “the taxpayers of Mississippi shouldn’t be subsidizing a plant that won’t even hire their own workers.”