Wal-Mart called the protests planned for this week a union-financed, union-orchestrated effort that did not represent how most employees felt. It said it expects small protests at just a few stores.

It is unclear how widespread employee support for OUR Walmart is; previous, smaller protests so far have been a mix of employees as well as union and community supporters.

Labor experts caution that the complaint, filed on Thursday, could be meant as a warning shot to discourage workers from participating since the labor relations board often takes months to make a ruling, but it nonetheless reflects how seriously the company has come to view a group that it had once dismissed as a nuisance.

William B. Gould IV, a Stanford University law professor and chairman of the labor board under President Bill Clinton, said the protests were more about employment conditions and retaliation against employees than a unionization drive.

"I don't see this translating into a great deal of success in terms of unionizing Wal-Mart or in terms of being particularly effective in improving conditions," Mr. Gould said. "But I must say if they've gone to the N.L.R.B. on this, that must show that Wal-Mart is really concerned." (Read More: Beyond Twinkies: Why More Workers are Striking.)

Since October, OUR Walmart has staged smaller strikes at individual stores, though none of those disrupted Wal-Mart's operations, the company said. OUR Walmart said that last month 88 workers at 28 stores did not report to work, in a job action.

OUR Walmart has also flown dozens of workers to Wal-Mart's headquarters and is encouraging religious leaders to hold a "Black Friday Prayer Vigil" to object to the company's treatment of workers.

Wal-Mart had initially brushed off these actions as inconsequential public relations efforts. But it is taking them more and more seriously, sending a memorandum advising managers how to deal legally with protesters and warning some union-friendly groups that they might face arrest if they trespass during the protests.

All this points to an increasingly fierce contest between Wal-Mart and labor groups that are bent on mobilizing and organizing the company's work force, with a near-term goal of pressing for higher wages and a longer-term goal of emboldening workers to demand a union.

"You are going to see unprecedented activity from now and going into Black Friday," said Dan Schlademan, a principal organizer of the events and director of Making Change at Walmart, an affiliate of the United Food and Commercial Workers Union. (Read More: Dear Bargain Hunters: Don't Be a Turkey on Black Friday.)

The food and commercial workers union has made Wal-Mart a target because the company has helped put many unionized supermarkets out of business and helped push down wages at many competitors. Wal-Mart, moreover, has vigorously resisted unionization drives, closing a store in Canada after workers there voted to unionize and arranging to have outside suppliers provide prepackaged meat after the butchers at a store in Texas voted to unionize in 2000.

The food workers union has been spending heavily on this push, paying more than $50,000 for hotel rooms near Wal-Mart's headquarters last year when it sent employees and representatives to company events, according to a filing with the Labor Department.

In this week's planned events, OUR Walmart, which stands for Organization United for Respect at Walmart, is enlisting a broad range of allies, arranging fliers and letters that community, church and civil rights groups can use to publicize the Black Friday protest. OUR Walmart has even prepared remarks that it is suggesting members of the clergy might use in prayer, "to call upon the world's largest corporation to treats its workers with justice and fairness."

Many of those workers assert that Wal-Mart pays poverty-level wages, assigns too few hours a week and retaliates against protesting employees.

"I will be protesting because there has been retaliation from the company — they have fired people, they have reduced people's hours for speaking out," said Greg Fletcher, an electronics department employee at a Walmart in Duarte, Calif.

David Tovar, a spokesman for Wal-Mart, said the company prohibits retaliation, and respects the rights of associates to express their views. But, he added, "if people repeatedly have unexcused absences, if they purposefully disrupt the store, or create an unsafe working condition for our customers and associates, those issues will be addressed" in accordance with company employment policy.

In the filing with the labor board, the company said that the continuing protests were illegal because under the National Labor Relations Act, a union seeking recognition can picket for a maximum of 30 days. After that, it must either stop picketing or take a formal unionization vote. The company says the United Food and Commercial Workers Union is behind the protests and has exceeded the 30-day limit.

"Many of these ongoing tactics being orchestrated by the U.F.C.W. are unlawful," Mr. Tovar said. "The United Food and Commercial Workers Union and its subsidiary, OUR Walmart, have been conducting illegal pickets and other demonstrations for several months now, clearly beyond what the law allows."

Officials with the union and OUR Walmart say the demonstrations and picketing aim to protest what they call illegal labor practices by Wal-Mart, specifically retaliating against protesting workers, and in no way aim to seek union recognition, as the company asserts.

The two labor groups insist the protests are being sponsored not by the union, but by OUR Walmart, which they say is an independent group.

Both groups acknowledge that the food and commercial workers provided guidance and financial support to OUR Walmart when it was founded last year. Indeed, in a 2011 filing with the Labor Department, the union described OUR Walmart as a "subsidiary organization."

But Jill Cashen, a union spokeswoman, said that OUR Walmart had "grown and gained independence" and attracted thousands of members since that time, and that it would not be listed as a subsidiary organization in the union's filing for 2012.

Angela B. Cornell, director of the labor law clinic at Cornell Law School, says the company probably knows that the labor board usually takes weeks or months to act, making it unlikely that Wal-Mart could obtain an injunction by Friday. She said she suspected the filing was more likely aimed at warning employees about engaging in what the company maintained was illegal picketing. A letter to the union from Wal-Mart uses the word "illegal" three times, though she said a one-time walkout like this was generally protected under the National Labor Relations Act.

"This was a strategic maneuver on their part," she said of the company.