Walmart workers and their supporters are launching protests in stores in 15 cities across the US today, as part of a small but vociferous movement to raise wages and improve conditions for some of the nation's lowest paid workers.

The action follows strikes last week by fast food workers demanding a higher minimum wage and a civil disobedience action in Washington DCin August, where a coalition of Walmart workers and unions called for a minimum annual wage of $25,000 and the reinstatement of 20 employees they claim were illegally fired by the company after strikes in early June.

OurWalmart, a union-backed members group, says it has filed more than 100 unfair labour practice charges against Walmart with the National Labor Relations Board, including 20 illegal terminations and 80 disciplinary actions. The board said it was looking into "several cases".

Walmart have denied any wrongdoing.

Thursday's actions will include a march in Los Angeles and a rally outside the Four Seasons hotel in San Francisco, where Marissa Mayer, Yahoo CEO and Walmart board member has a penthouse apartment. InNew York, a petition will be delivered to the headquarters of Williams Capital Group, whose chairman, Christopher Williams, is a Walmart board member. Other actions are expected in Washington DC, where a bill was recently passed by the city council to pay workers for large retailers $12.50 an hour. Workers will also protest in Chicago, Boston, Sacramento, Miami and Dallas, according to organisers.

Last week's fast food workers were aimed at raising the federal minimum wage, which has not altered in four years, from $7.25 to $15 an hour. Walmart workers earn on average $8.81 an hour, according to OurWalmart. Walmart says its "average full-time hourly associate" makes $12.83/hour.

The protests by workers represent a tiny proportion of Walmart's 1.3 million workforce, but organisers say Thursday will mark the largest action since protests last November when, they claim, 400 workers walked off their shifts.

The scattered protests follow increasing scrutiny of the retailer in recent months, after reports of international bribery allegations in Mexico, and questions about factory safety among its global suppliers after 1,000 garment workers were killed when a building collapsed in Bangladesh. Concerns over the retailer's employment practices were also cited by two European pension funds, in their decisions to remove their investments in the retailer. Dutch pension managers PGGM and Mn Services declared in June that they would divest from Walmart, claiming that the company does not treat its employees in accordance with international standards of freedom of association and has failed to respond to their concerns.

The alleged terminations and disciplinary actions relate to a demonstration in June by more than 100 workers who travelled to Bentonville, Arkansas, to take part in a rally at Walmart's shareholders meeting, according to OURWalmart.

Raymond Bravo, 36, from San Pablo, California, who earned $10.25 an hour as a janitor for a Walmart's Richmond Hilltop Mall store in California, working 30 hours a week, said he was fired from his job after taking part in the strikes and demonstrations in June. Bravo, who is single, said he became active in the protests after seeing his coworkers run out of money at the end of each month. "Three days before payday I would see people who normally eat not eating," said Bravo. "They would be borrowing money from each other."

Bravo said he took part in a strike, from 29 May to 12 June, not only for higher wages but in order to have a voice in the workplace. "I was fired for going on strike, for speaking out," said Bravo, who worked at the Richmond store for two years. "They told me it was for missing days. But in America, you can go on strike and you are protected. I hope the government forces Walmart to give me my job back because it will give others the courage to join up."

Lawmakers, including Keith Ellison, of the Congressional Progressive Caucus have condemned the terminations describing them as "completely unjust and illegal".

Dan Fogelman, a spokesman for Walmart, denied any wrongdoing and said that he did not believe the work stoppages were "protected activities" under labour laws. He said: "Walmart strives to follow the law in everything that we do. We don't believe that these hit-and-run intermittent work stoppages that are organised by unions are protected activities."

Fogelman said that it was "disrespectful for managers and customers alike" for workers not to turn up for work for several days. However, he said that Walmart's decision to discipline its employees had "nothing to do" with the strikes.

Those who were fired had violated "our attendance policies under a multi-step progressive disciplinary process" laid out in a manual, he said. Employees are given "coachings" when they violate policy, including, for instance, when the cash in the cash register comes up short, Fogelman said. Multiple "coachings" can lead to discipline or termination.

Fogelman refused to provide the manual or other details of its disciplinary process, saying it was not a public document.

Professor John Logan, director of labor studies at San Francisco State University, described Walmart as "a poster child for everything that is wrong with US labor law and employment relations."

Logan said: "The workers terminated by Walmart may eventually get justice, but the NLRB's processes are notoriously slow – it sometimes seems to move at glacial speed – and its remedies for unfair management practices are extremely weak. Walmart has consistently used legal and illegal practices to silence employees who dare to speak out."

Gregory King, of National Labor Relations Board, said it was "looking into several cases" involving Walmart but said the board was unable to comment on specific cases and would not provide further details.

The protests by low-paid workers have drawn fresh scrutiny to the issue of workers rights and the minimum wage. President Barack Obama has backed a minimum wage raise to $9.

Tom Perez, the Labor secretary, spoke favourably last week about the Living Wage Bill passed by the city council in the District of Columbia, that would require large retailers to pay workers $12.50 an hour, a 50% premium over the city's minimum wage.

But Walmart has warned that it would not go ahead with plans to build three stores in Washington DC if the measure becomes law and said that it jeapordises stores already under construction.

A congressional report published in May, found that, in Wisconsin, Walmart had more workers enrolled in the state's public health care program in the last quarter of last year than any other employer. The report, The Low-Wage Drag on Our Economy, was produced by Democrats with the House Committee on Education and the Workforce. The committee says it chose Wisconsin because the state's data "appears to be the most recent and comprehensive." The paper is an updated version of an earlier report by the same committee in 2004, which at the time estimated that a 200-employee Walmart store could account for $400,000 in public assistance for workers.

Fast food workers interviewed by the Guardian last week also reported being given benefits such as food stamps, in order to subsidise their low wages.