CEDAR RAPIDS, Iowa — Federal immigration agents in northeastern Iowa raided the world’s largest kosher meatpacking plant yesterday, arresting more than 300 people and housing many of them at a converted fairgrounds.

The raid of the Agriprocessors Inc. plant in Postville was the largest in state history and had been planned for months, U.S. Attorney Matt M. Dummermuth said.

The raid, one of several conducted at meat-processing plants around the country in recent years, was aimed at seeking evidence of identity theft, stolen Social Security numbers, and people who are in the country illegally, an ICE spokesman, Tim Counts, said.

Authorities arrived at the Agriprocessors plant about 10 a.m. and presented company officials with search warrants. Agents asked to speak with all the employees, and plant officials cooperated and shut down their operations.

Agriprocessors did not immediately return messages seeking comment.

Of those arrested, 44 were released for humanitarian reasons, primarily because they must care for children. They were ordered to report to court later.

Others arrested were held in local jails or driven by bus about 75 miles to temporary housing at the National Cattle Congress fairgrounds in Waterloo. Federal authorities previously leased the fairgrounds and have turned it into a secure center.

Sister Mary McCauley, a Roman Catholic nun at St. Bridget’s Catholic Church in Postville, said family members of plant workers came to the nearby church in tears after immigration agents arrived in buses, vans, and two helicopters.

“The people right now are hearing and seeing the helicopters,” Sister McCauley said yesterday morning. “They are just panic-stricken and very frightened and some of them are coming to the church as a safe haven.”

Sister McCauley said she went to the plant to help provide information and assist workers but was not allowed to get close.

“Some of the people that are going to be detained are up against a fence and now they’re tying their hands,” she said.

Many of the plant workers are Hispanic, mostly from Mexico and Guatemala, she said.

Mr. Counts, the ICE spokesman, said a toll-free telephone number had been set up to assist family members of those arrested who have questions about their detention status and the removal process.

Asked about the raid during a news conference yesterday, Governor Chet Culver said both illegal immigrants and companies that knowingly hire them should be prosecuted.