By SUSAN SAULNY

In the biggest workplace immigration raid this year, federal agents swept into a kosher meat plant on Monday in Postville, Iowa, and arrested more than 300 workers.

The authorities said the workers were suspected of being in the United States illegally or of having participated in identity theft and the fraudulent use of Social Security numbers.

A spokesman for Immigration and Customs Enforcement would not say how many people had been rounded up beyond the initial 300 or whether the management and owners of the plant, AgriProcessors, would face criminal charges.

The plant has 800 to 900 people and is the country’s largest producer of meat that is glatt kosher, widely regarded as the highest standard of cleanliness.

The plant shut temporarily.

The agents set up a perimeter around the 60-acre plant, in northeastern Iowa, and entered on the morning shift, carrying out two search warrants, federal authorities said. An affidavit filed in court before the raid by the Homeland Security Department cited “the issuance of 697 criminal complaints and arrest warrants against persons believed to be current employees” and to have acted criminally.

The affidavit said a former plant supervisor had told investigators that a methamphetamine laboratory had operated at the plant and that some employees had carried weapons to the plant. The former supervisor, the affidavit said, estimated that 80 percent of the employees were in the United States illegally.

A spokesman for Representative Bruce Braley, Democrat of Iowa, said the number of arrests was expected to increase, perhaps even double, as the investigation continued.

Federal officials leased an expansive fairground area in nearby Waterloo to process and house the arrested workers. Among people at the fairgrounds and in Postville, “there is a lot of fear,” said Prof. Mark A. Grey, who focuses on immigration at the University of Northern Iowa.

“It’s absolutely devastating to the local economy,” Professor Grey said.

In a news release, Matt M. Dummermuth, the United States attorney for the Northern District of Iowa, called the sweep “the largest operation of its type ever in Iowa.”

Federal authorities have been conducting workplace raids across the country in recent years, with the pace accelerating since the failure of immigration legislation last year in Congress.

The raid had been planned for months and was conducted in coordination with local law enforcement, according to the news release, released jointly by Claude Arnold, special agent in charge for the ICE regional office in Bloomington, Minn.

Calls to AgriProcessors, a global giant in the kosher meat market and the major employer in Postville, a town of 2,200 people, were not answered. A lawyer for the plant did not return a call.

According to a company Web site, Aaron Rubashkin, whose family controls the plant, bought a defunct meat factory in Postville in 1987 and turned it into the present plant.

According to Menachem Lubinsky, the editor of Kosher Today and a marketing consultant, AgriProcessors provides 60 percent of the kosher retail meat and 40 percent of the kosher poultry nationally, and most retail chains depend on it for supply. Mr. Lubinsky said the company was also the sole American packing plant whose products are accepted in Israel.

The raid was not the first moment in the national spotlight for the plant. In 2004, it was asked to change its slaughtering methods after an animal rights group secretly documented workers cutting the throats of living steers and letting them bleed to death.

The company has also been a target of environmental pollution complaints.