The march was called to protest working conditions in the plant, owned by Agriprocessors Inc., and to call for Congressional legislation to give legal status to illegal immigrants. The four rabbis, from Minnesota and Wisconsin, attended the march to publicize proposals to revise kosher food certification to include standards of corporate ethics and treatment of workers.

The march drew a counterprotest by about 150 people, organized by the Federation for American Immigration Reform, which opposes illegal immigrants and proposals to give them legal status.

At one point, tension surged as the two sides shouted slogans at each other through bullhorns from opposite sidewalks of the main street of this town with a population of about 2,200. The marchers said, “Stop the raids!” Protesters across the street responded, “Illegals go home!”

No incidents of disorder were reported by the police.

The debate over kosher standards has intensified since the May 12 raid at the plant, in which 389 illegal immigrants, the majority from Guatemala, were detained. Reports by many of those workers of widespread labor violations in the plant have been prominent news in the Jewish media, provoking discussion of whether Jews should buy meat and poultry products made there.

Agriprocessors, owned and operated by Aaron Rubashkin and his family, is the largest kosher plant in the United States. Its products, sold as Aaron’s Best and Rubashkin’s, among others, dominate the nation’s market for kosher meat and poultry.

The plant had been cited for state and federal labor violations before the raid, including inadequate worker safety protections and unpaid overtime. Since the raid, immigrants under 18, the legal age in Iowa for working on a meatpacking floor, have said they worked long hours at Agriprocessors, often at night.

Agriprocessors’ beef and poultry are killed and packaged using procedures specified by strict Jewish dietary laws, and are certified by rabbis who are recognized authorities on kosher food.

In 2006, after reports in The Forward, a Jewish newspaper, of harsh working conditions at Agriprocessors, a commission of inquiry organized by Conservative Jewish leaders criticized the plant’s operations and called for more safety training and increased inspections by state labor officials.

A member of that commission, Rabbi Morris Allen of Mendota Heights, Minn., proposed a new system of kosher certification that would include consideration of working conditions in plants where the food is produced.

Rabbi Harold Kravitz, from the Adath Jeshurun synagogue in Minnetonka, Minn., said on Sunday that the health and safety issues raised by the commission did not appear to have been addressed. Speaking to the rally on a dusty driveway in front of the plant, Rabbi Kravitz said that Jewish laws governing the kosher processing of animals should not be separated from Jewish ethical principles.

“Proper business conduct and treatment of workers also are important Jewish values,” Rabbi Kravitz said.

He and several Jewish community activists met on Sunday morning here with Chaim Abrahams, a top manager of the plant. Aaron Goldsmith, a Postville resident who participated in the meeting, said Mr. Abrahams reported that about 360 of the arrested workers had received all payments that they were owed and that Agriprocessors was making weekly deliveries of food to about 30 immigrant families in Postville.

Although Agriprocessors executives have largely avoided speaking to the news media, Getzel Rubashkin, 24, a grandson of Aaron Rubashkin, emerged from the plant and approached the rally.

“There’s no argument here,” said Getzel Rubashkin, who said he works in the plant but was not a representative of Agriprocessors and was speaking for himself. Agriprocessors managers, he said, “treat their workers well and they pay their workers well and there is no other policy.”

“The company is not on the other side of any of these people,” he said, referring to the immigrants lined up behind banners across the street from the plant.

Getzel Rubashkin said a large number of illegal immigrants had been hired because they presented identity documents that he called convincing forgeries.

“The high number of illegal people who were working here is more a testimony to the quality of their deceit, of their papers,” Getzel Rubashkin said. He said the company did not criticize immigration authorities for the raid.

“Obviously some of the people here were presenting false documents,” Getzel Rubashkin said. “Immigration authorities somehow picked it up and they did what they are supposed to do, they came and picked them up. God bless them for it.”

On Postville’s main street, the protesters opposing the immigrants’ march praised Iowa federal prosecutors, who convicted 297 illegal immigrant workers from the plant, most on criminal document fraud charges.

“It’s a felony when you take someone’s identity, and we think that needs to be out there when you talk about the supposed injustices against undocumented workers,” said Susan Tully of the Federation for American Immigration Reform, an organizer of the counterprotest.

Like the marchers, the protesters were also angry at Agriprocessors managers. To date, the only managers arrested were two floor supervisors, on immigration harboring charges.

“It’s cheap labor, that’s what they’re getting away with,” said Ruthie Hendrycks, 48, of a group called Minnesotans Seeking Immigration Reform. “I want to see these employers that hired children and illegal aliens do serious jail time.”