Luis Zavala knew something had gone wrong when he saw the gun pointed at him.

The 45-year-old construction worker in Louisiana, and about two dozen others in his crew, had gathered in the small town of Kenner, on the outskirts of New Orleans, where he believed they were about to be given unpaid wages owed by their boss.

The move was set to bring an end to a bitter dispute, and their employer had told them to gather at a car park in front of an apartment complex. But instead of meeting him, they were surrounded by armed police and officers from Immigrations and Customs Enforcement.

Zavala, who was waiting in his car when the violent raid began, found himself dragged outside his vehicle, cuffed and laid out on the ground. "They came very aggressively, with their guns pointed. They threw me on the floor. They put their foot on my back. It was very terrifying," Zavala told the Guardian.

Zavala and his colleagues had just run up against a brutal truth that affects more than 11 million undocumented migrants who work in the shadowy low-wage depths of the American economy: speaking up against an employer who abuses you can get you arrested and deported.

Millions of workers like Zavala toil in industries like construction, casual day labour, agriculture or the food industry across America and, as Zavala and many others have found, standing up and complaining can result in an employer reporting them to the immigration authorities.

Experts point out that some employers are all too eager to take on undocumented workers and exploit them for their willingness to work long hours for low pay. If no one complains, questions about immigration papers are rarely asked. But if problems do arise – such as being injured on the job or workers demanding better pay or access to a union – a swift phone call to the police or ICE will result in the difficult employees being deported.

"Employers use this as a huge club against workers who stand up for themselves," said Rebecca Smith, an expert at the National Employment Law Project, which campaigns on various worker abuse issues in the US.

Smith recently co-authored a study of the phenomenon for NELP called Workers Rights on ICE, which chronicled many recent cases where the threat of deportation had been used against workers who tried to unionise or complained about safety standards or had their wages stolen. But Smith believes that because most victims are either deported or afraid of being deported the vast majority of incidents taking place in America simply never come to light. "It is the tip of an iceberg," she said.

But some do speak out. One is Felipe Villareal, a former car-wash worker from Mexico who has been in the US since 2000. Like many undocumented workers he was willing to do a job that American workers did not want and certainly not for such little cash. Villareal worked amid the chemical fumes and hot water of a mechanised car wash in Culver City, California, often just for tips. "There was a lack of safety protection. They use a lot of toxic chemicals that get on your skin and your lungs," he said.

Villareal was fired when he complained about his low wages, but instead of saying nothing, he launched an unfair labour practice complaint that forced his reinstatement. But it was then that his employer suddenly decided he needed to go through the Department of Homeland Security's E-verify system in order to get his job back. Villareal, who declines to discuss his immigration status, believes there was only one reason for bringing in the new rule at a company that until then had asked few questions. "I believe that they used the immigration threat for people like me who spoke out. It gives employers power over their workers," he said.

Sometimes that power is brutally explicit. In Austin, Texas, Antonio Campos Lozano and five of his fellow construction workers had gone unpaid by their boss. They had worked night shifts doing building work in a local mall and then often worked through the day elsewhere around the city for the same man.

"Sometimes you don't go home. You just get ready for the next shift. We work for our families. That's the main reason. We have bills to pay and families to support," he said.

But then their boss refused to pay them, and the crew reported the local businessman to a workers' rights group which took legal action.

Astonishingly, Lozano then started getting text messages from the man threatening deportation to the entire crew. "I am going to do whatever it takes to have them sent back to Mexico," read one message sent to Lozano and seen by the Guardian. Lozano is in the US legally, but the employer was explicit about the rest of the crew who were undocumented. "Your [sic] the only one that won't get sent back. Tell that to them," another text stated.

The impact of such threats on the mental wellbeing of undocumented workers is chilling. Lozano says that his five friends, who already struggled to make ends meet despite working such long hours, now turn down valuable work if it does not come from someone they trust. "They are afraid to trust anyone they do not already know in case that happens again. He did it to make us feel like nothing. He thought we are not going to talk to anybody," Lozano said.

Maria Guadalupe Escobar certainly knows that feeling. Last year she and other workers at a seafood packing plant in Orange County, New York, filed a lawsuit against their employer for violating labour standards. A few months later, Escobar was stopped in her car by DHS agents and later arrested and transferred into immigration custody.

"The employer knew we were undocumented workers. He hired us knowing that, and took advantage of us by not paying us overtime. He knew we were vulnerable and desperate. He never expected us to stand up for our rights, our human rights, and then when we did, he used his connections to send immigration after me. He targeted me," she told the Guardian.

Escobar, who is trying to fight being deported, said her experience has been deeply traumatising. "I feel like I'm nothing. Like the dirt. Like I've been thrown on the ground. I've already sacrificed so much, given so much of myself, working for such low wages, and then to be treated like this. It's too much," she said.

One area of hope might lie in current efforts towards immigration reform which look set to agree on some sort of path towards citizenship for undocumented workers already in the country.

But experts warn things may not be so straightforward for low-wage workers, even if immigration reform is passed. They say it is vital to make sure any new law includes labour protections, otherwise the exact same conditions that exist now – a cheap pool of easily exploitable immigrant labour open to unscrupulous employers – will simply develop again.

"We have to make sure we don't just recreate this system we watch as an underclass develops that can be exploited," said Smith.