Eight seafood plant workers are calling on Walmart to drop a supplier which they say forced them to work up to 24 hours a time and threatened their families with violence.

The workers, who are all from Mexico, were employed to process crawfish by CJ's Seafood in Louisiana, under conditions which an independent report has described as "rivalling any sweatshop in China or Bangladesh".

Separately, a group of warehouse workers have filed a complaint against Walmart and a PR firm associated with the supermarket giant, after a public relations officer was found to have posed as a student journalist to interview activists campaigning against working conditions in warehouses supplying big retailers.

The CJ's Seafood workers say they were locked inside the crawfish processing plant during periods of peak demand. Their families in Mexico were threatened, according to one worker, while they were warned they faced deportation if they did not acquiesce to their employer's demands.

The eight, with the support of the not-for-profit National Guestworker Alliance, lodged a complaint with the wage and hour division of the department of labour this month, claiming that the company violated the fair labor standards act by failing to pay overtime and failed to keep accurate records as required by the act and by employers of workers on H2-B visas.

The workers lodged a separate complaint with the occupational safety and health administration (OSHA), claiming the working conditions at CJ's flouted the Occupational Safety and Health Act of 1970.

Separately the workers filed a charge of discrimination with the equal employment opportunity commission – the first step in the process of bringing a discrimination lawsuit against an employer.

The complaint alleges that the eight were discriminated against on the basis of race, color, national origin and retaliation, stating that Latino workers "are forced to work longer and less desireable hours" than other people of colour, and alleging that the workers were not paid "according to the terms of their contract".

The department of labour confirmed two investigations, by the wage and hour division and the occupational safety and health administration, were ongoing. The EEOC is prohibited from discussing investigations by federal law.

On Tuesday the eight workers delivered a petition with 130,000 signatures to the New York home of Walmart board member Michelle Burns, while they travelled to Washington DC on Wednesday to meet with members of the Senate appropriation committee.

'They locked the doors so we couldn't take breaks'

The workers are hoping to pressure Walmart into revoking their contract with CJ's Seafood as well as launch an investigation into standards at the plant.

Ana Diaz, one of those on strike, said that a supervisor at the plant warned workers that they would not want him as an enemy.

"He said he had contacts with good people and bad people, and that they could find us no matter where we went," Diaz told the Guardian.

Around 40 Mexicans are employed at CJ's Seafood, according to the alliance, with the eight campaigners currently on strike against conditions.

Diaz, who travelled to Louisiana from her home in Tamapaulipas, in north-east Mexico, said workers at the plant were involved in either peeling or cooking crawfish. Those peeling were mostly women, who worked from 2am until 5pm. Those cooking the fish tended to be men, who Diaz said were forced to work up to 24 hours at a time.

"On two occasions they locked the doors so we couldn't take breaks because they wanted us to produce the crawfish in a shorter time. He said that if we took breaks then he would lose a lot of money."

"We were afraid because he does know where many of us live. He knows where our family members live; he has our addresses and information. That's when we felt the most locked in and the most abused."

Neither Walmart nor CJ's Seafood responded to requests for comment, although Walmart spokeswoman Megan Murphy told the Daily Beast: "Following our investigation, as well as investigations by the department of labour and OSHA, at this time we are unable to substantiate claims of forced labour or human trafficking at CJ Seafood."

Given the investigations by the department of labour began less than three weeks ago, however, it is likely to be some time before they conclude. A spokesman would not comment on the status of the probes, except to say they were still ongoing.

An independent investigation by the Worker Rights Consortium (WRC), originally set up by university administrators, students and labour rights campaigners to prevent sweatshops being used to manufacture university clothing but which now also conducts investigations into broader working conditions, found that "the totality of the abuses taking place at this employer constitute forced labour under US law".

"The conditions at this Walmart supplier are among the worst we have encountered, rivaling any sweatshop in China or Bangladesh," said WRC's executive director Scott Nova.

Jacob Horwitz, lead organiser, said the system of bringing temporary workers into the US "really amounts to creating a whole class of workers without access to the same rights as workers who are [already] in the country".

Some 130,000 people have signed a petition supporting Diaz and her colleagues on Change.org calling on Walmart to sign an agreement to guarantee civil and labour rights for guestworkers in the US, which Diaz and others delivered to Burns's address in lower Manhattan on Tuesday.

It has been a difficult week for the supermarket giant. Last Thursday a public relations officer working for a firm contracted by Walmart was revealed to have posed as a student journalist to interview a labor group highlighting tough working conditions in warehouses that supply by big retailers.

Stephanie Harnett, a publicist for Mercury Communications, interviewed an activist from Warehouse Workers United (WWU) under the assumed name "Zoe Mitchell".

On Wednesday WWU filed complaints against both Walmart and Mercury Communications for what it called "illegally spying on workers".

"Warehouse workers risk retaliation every time we speak to the media and to learn that the company was hiding behind a fake reporter makes me really mad," said Santos Castaneda, the worker interviewed by Harnett.

The complaint was filed with the regional office of the national labor relations board, which prohibits spying on workers, WWU said.