WASHINGTON -- A crackdown on illegal immigrants in the workplace -- the largest ever by federal authorities -- is drawing mixed reviews, even from some who demand tighter borders.

Advocates for more enforcement weren't impressed, immigrant rights groups howled in protest, and labor-union lawyers filed court papers claiming violations of the workers' constitutional rights.

Federal officials said Wednesday that their raids on meatpacking plants in six states uncovered a "disturbing front" in the battle against illegal immigration -- identity theft as a tool to obtain employment.

Homeland Security Secretary Michael Chertoff said the widespread raids against Swift & Co., a Greeley, Colo.-based meat processor, were part of an ongoing effort to get tough on immigration violators at the workplace.

Some 5 percent of the 1,282 arrests resulted in identity theft charges, and Chertoff said the number probably would rise as agents look into more workers' cases and into rings of vendors who steal and sell illegal documents.

"Violations of our immigration laws and privacy rights often go hand in hand," Chertoff said. "Enforcement actions like this one protect the privacy rights of innocent Americans while striking a blow against illegal immigration."

Observers on both sides of the immigration debate were somewhat skeptical, calling Chertoff's crackdown on identity theft a new refrain in an old song.

Advocates for more enforcement said they had little faith that the Bush administration would keep up the raids because the president supports a guest worker program and a path to citizenship for some of the 11 million illegal immigrants in the country now.

"This is a little bit of enforcement to create a cover for amnesty," said Steven Camarota, research director at the Center for Immigration Studies, which favors vigorous immigration enforcement.

On the other side, immigrant rights groups said the raid terrorized thousands of workers and their families but did little to the employers or the people who sold the workers the documents in the first place.

"We've been doing raids for 20 years, and the immigration problem is soaring," said Frank Sharry, executive director of the National Immigration Forum, an immigrant advocacy group. "You can't restore the rule of law until you respond to the law of supply and demand."

Attorneys for the United Food and Commercial Workers filed papers in federal court in Denver on Wednesday saying the arrests of the Greeley workers violated their constitutional rights. The filing claims that those arrested are being denied access to lawyers and that their whereabouts are unknown.

Chertoff said the raids highlighted a need for the guest worker program, which would cut down on the demand for illegal documents.

He also asked for more tools from Congress to allow his agencies to detect when Social Security numbers are being used by multiple people. Swift participated in a program known as Basic Pilot, which snags false Social Security numbers but doesn't identify stolen identities.

An effort died in Congress this year to create a guest worker program and a path to citizenship for illegal immigrants. Instead, Bush signed a bill this fall authorizing a 700-mile fence at the U.S.-Mexico border.

Colorado Sen. Ken Salazar, a Democrat who pushed for the guest worker bill, said he has talked with Democrats and Republicans who are open to reviving the legislation within the first six months of the new Congress.

"If you're going to address the problem, it needs to be addressed in a comprehensive manner," he said. "It breaks my heart when I see the pictures that I saw in this morning's paper of scared children. That's not the America of law and order ... of compassion, that I know."