If Karl Marx could visit the freezing, rickety shack by Bulevar Independencia, he might reconsider the inevitability of labour trumping capital.

There is gravel underfoot, the tarpaulin roof flaps in the winter wind and frost dampens the cardboard walls. Traffic roars past, oblivious, and the handful of occupants inside subsist on donated beans and tortillas.

Tattered banners proclaiming “libertad sindical” (union freedom) and “justicia a la clase obrera” (justice for the working class) adorn the walls. This is the nerve centre of Ciudad Juárez’s worker rebellion.

Opposite the shack – behind security cameras, guards and gates – is the factory that fired them after they tried to form a union.

Lexmark, a Kentucky-based corporate leader in laser printers, is worth around $2bn and has the support of Mexico’s political establishment and apparently also its media and Catholic hierarchy, notwithstanding Pope Francis’s visit to Juárez on Wednesday.

The elites wish to snuff out defiance and stop rebellious contagion spreading across this industrial city, according to those inside the shack.

“We’re living on charity, and it’s tough, but we’re still here,” said Susana Prieto Terrazas, a lawyer representing the protesters, as she huddled by a wood-burning stove. “We’re going forward. This is a system of modern slavery and we have to fight.”

It has been a time bomb. This protest is an enormous opportunity Elizabeth Flores, labour rights advocate

The fight here is largely without precedent.

Ciudad Juárez, a gritty city of around one and a half million across the border from El Paso, Texas, is a global economy workshop. About 300 factories with headquarters in the US, Europe, China and elsewhere employ about 300,000 Mexicans. There are virtually no independent unions and, until now, little sign of worker mobilisation. Historically low wages are changing that.

“Conditions have become insufficient for survival,” said Elizabeth Flores, director of Pastoral Obrera, a labour and human rights advocacy group. “It has been a time bomb. This protest is an enormous opportunity.”

The shack occupied by former Lexmark workers in Ciudad Juárez. Photograph: Rory Carroll/The Guardian

The factories, known as maquiladoras, form a sprawling landscape of low-slung, windowless buildings lining highways, usually beige coloured and anonymous looking save for the logos of companies with names like Columbus Industries, Align Technology, Salter Labs, Matrix, Victory Packaging, and Éco Pak.

They make televisions, air conditioners, medical scopes, battery cases, printers, plugs and cables – stuff that fills homes and offices and clinics around the world.

They operate under a system that lets them import tariff-free raw materials for manufacturing and then export the finished products. For more than 50 years this industrial hub has provided jobs to workers, especially women, who flock here from all over Mexico. It continued humming amid the narco-fuelled violence of the past decade.

The problem, say protesters, is that the jobs have become a form of serfdom: grim conditions – long hours, frenetic pace, management intimidation – and shrivelled wages. The peso’s plunge against the dollar has ratcheted up the cost of living.

“In the factory speed is everything – you have to be fast. But no matter how much you work you barely have enough to eat,” said Miriam Delgado Hernández, 37. The single mother said she packed ink cartridges 48 hours a week for six years at Lexmark’s plant. She earned 670 pesos ($35) a week – or $7 a day.

Mexico’s daily minimum wage is 70.10 pesos ($3.70), the lowest relative to average wages in the 34-member Organisation for Economic Cooperation and Development.

The protest flared last October at four companies – Lexmark, Foxconn, ADC/Commscope and Eaton. In addition to allegations of sexual harassment, workers accused management of reneging on a promised wage increase of around 6 pesos – $0.32 – per day.

There were hunger strikes, marches, leafleting and the erection of the shack outside Lexmark, an Occupy-type symbol of revolt manned even during freezing desert nights. In December, about 700 Lexmark workers went on strike. There was a giddy sense of momentum.

Then the pendulum swung.

Lexmark fired about 75 workers who had submitted a formal request to start a union. Their names were supposed to be known only to the state government’s Board of Conciliation and Arbitration. “They gave the list to Lexmark,” said Terrazas, the lawyer.

The board has denied that, saying it has no idea if or how the names leaked.

The factories are like fortresses. If I do manage to give someone a flyer, they look frightened Susana Prieto Terrazas, protest lawyer

In an email to to the Guardian, Jerry Grasso, a Lexmark spokesman, also denied it, saying the company fired the 75 for “workplace disruption”, not for trying to start a union. “While we will not specifically go into details about the disruption … it is fair to say that Lexmark has managers on the plant floor during all shifts. It would be fairly obvious to any manager, or employee, which co-workers were disrupting work.”

Grasso added that in each market, Lexmark benchmarks salaries against similar jobs and roles. “We take our values at Lexmark very seriously, including the value of mutual respect. We embrace individual differences and listen to all voices. We’ve engaged with our employees in the plant, as we do with our employees globally. We are committed to ensuring that Lexmark continues to be a rewarding place to work in Juárez.”

People line a street in Juárez ahead of Pope Francis’s upcoming visit. The pope is expected to focus on poverty and social justice. Photograph: Jose Luis Gonzalez/Reuters

The protesters allege the company uses intimidation to keep remaining employees in line, including using trucks to block the shack from view when they file in and out of the factory. “The factories are like fortresses. If I do manage to give someone a flyer, they look frightened,” said Terrazas.

Workers who remain at their jobs agreed pay was too low but said they could not afford to strike, let alone endanger their livelihoods. “I have six children. I can’t lose a day,” said Adrián Calderón, 36.

Local newspapers and television stations have virtually ignored the protests, allegedly at the behest of the factory owners and the government, which account for much of advertising revenue.

Pope Francis is expected to focus on poverty and social justice during his five-day visit to Mexico, which climaxes in Juárez on Wednesday, but Mexico’s conservative Catholic hierarchy has also ignored the protests. No Catholic cleric has visited the protest site.

Business and political leaders hope to use the pope’s visit to project the frontier city as a fast-growing industrial dynamo which has overcome the mayhem of the narcotics wars.

The protesters represent a tiny fraction of an otherwise content workforce, said Guillermo Dowell, a spokesman for the state governor. “They’re trying to make a lot of noise but the impact is actually small. The factories pay well above the minimum wage.”

However, local economists and business analysts warned the magazine Revista Net of a tinderbox of poverty and desperation. “If the companies don’t respond adequately, it could generate instability,” said Erika Donjuan.

Flores, the human rights advocate, said the protest will not produce immediate change but could lay the groundwork for a powerful labour movement.

In his 1848 Communist Manifesto, Marx predicted capitalist contradictions would topple the factory-owning class. “Its fall and the victory of the proletariat are equally inevitable.”

It doesn’t feel like that in the shack by Bulevar Independencia. The fired workers have mounting bills and no salary, and they feel isolated. They survive on food donations from US churches and progressive groups, plus $7,000 raised by Miguel Juárez, an El Paso-based activist who set up a crowdfunding page.

Asked how long they could hold out, the workers paused. “As long as we can,” said Antonia Hernández Hinojos, bundled in a fleece and padded coat. The 45-year-old grandmother used to assemble several dozen fuses per hour for the Eaton corporation until it fired her in December.

The protesters selected the woman, known as Tonita, to run for mayor of Ciudad Juárez in the coming June election. “We need to raise our voice,” she said.

To challenge established parties and other independent candidates, Tonita needs to rouse factory workers, a sleeping electoral giant. It won’t be easy.

Tonita must clear several hurdles to register with the state electoral institute as well as circumvent media aloofness. Some reports of the mayoral race do not even name her, referring merely to “a worker candidate”.

Snuggled in the shack, Tonita smiled and raised a fist. “We’ll find a way.”