Employees do a quality inspection on a Volkswagen AG Jetta vehicle at the company’s assembly plant in Puebla, Mexico. (Susana Gonzalez/Bloomberg News)

— It wasn’t a planned strike, more an impromptu protest: A group of assembly-line workers at the Mazda car factory one spring morning just stepped away from their posts.

In addition to long, strenuous hours, they said, they endured constant taunts from one assistant manager, who also had allegedly sexually harassed an employee. They refused to work until they could air their grievances with his superiors.

“We showed up, but we decided not to work,” said Eder Capetillo, a 29-year-old worker who participated in the one-day halt in March. “We were asking for respect.”

Across Mexico’s economic landscape, it is cars as far as you can see. In a country burned out on beastly news — drug violence, political corruption, kidnappings — the auto industry is the beautiful princess. States are competing to seize the investment pouring in: Seemingly each month a new auto company is announcing billion-dollar expansion plans. In April, Ford said it would spend $2.5 billion to build engines and transmissions, while Toyota plans to put $1 billion into a new factory to build Corollas. Mexico is now the world’s seventh-biggest car producer, surpassing Brazil, and rising quickly.

“This is the strongest industry we have right now,” said Miguel Márquez Márquez, the governor of Guanajuato, where there has been more than $7 billion of auto industry investment just in the past three years. Car manufacturing accounted for a large portion of the state’s new jobs last year. University programs are churning out hundreds of engineers and teaching Japanese. More than 20 hotels are planned. “It is a virtuous circle: When the companies win, our people win.”

In the enthusiasm for Mexico’s auto boom — 3.2 million cars were produced here last year in 18 factories — the question of labor conditions often is overlooked. Industry analysts and experts say most of these jobs provide above-average employment for Mexicans, offering insurance, overtime and other benefits in state-of-the-art factories.

But the labor dispute at the Mazda factory is a reminder of the challenges that can crop up behind the gates of these mega-factories. In an environment where these auto jobs are in demand, workers say they have little recourse in conflicts with management. A few weeks after Capetillo and his colleagues complained, they were fired or forced to resign — 17 workers at a plant that employs roughly 5,000 people. The months since then, they say, have been a losing battle against unresponsive union representatives and an apathetic state government.

“They don’t treat you with humanity. It was exploitation in general,” said Ricardo Gutierrez, 32, who had spent two years at the plant before losing his job. “But there was nothing we could do.”

After their firing, there was a brief flare-up of publicity, with some of them publicly criticizing Mazda. But when the news conferences were over and the Facebook postings on pages with angry names such as “Mazda Modern Slavery” began to taper off, they found themselves largely out of options. They said their protest was legal and did not disrupt the day’s production. Mazda officials, they said, assured them they would not retaliate against them for raising concerns.

The state prosecutor’s office said it would investigate, but nothing has come of it. Márquez said that the problem should be resolved by Mazda but that the company “will have to be much more careful” in the future about employee concerns.

“What happened to them is totally illegal,” said Adrian Guerrero Caracheo, an official with a telephone workers union in Guanajuato who took up the Mazda workers’ cause. “They threw 20 people out on the street who were just asking for dignified work.”

The company disagrees. Refusing to work that day in March represented a “serious breach of the general policies of the company” and “put at risk the whole operation of our company,” a Mazda spokesman said in a statement. “Our labor conditions comply fully with the law.”

Mexico’s constitution does permit labor strikes in certain circumstances, and the tactic has been used by other unions, including auto workers. Public school teachers in the southern state of Oaxaca are on strike and not holding classes while they argue with the federal government over new rules requiring teacher evaluations. The union-led protests even involved cutting off the gas supply to filling stations the week before mid-term elections.

But several workers at the Mazda plant said that their union representative did not advocate on their behalf and that the local government will not stand up to such an important industry. Caracheo said he has started discussions about forming a new union to serve the booming automotive industry.

In general, industry experts say the labor conditions at auto factories tend to be better than in many other industries. These jobs serve as a model for the type of formal, higher-tech, tax-paying employment that Mexico wants to cultivate. But part of the reason Mexico is so attractive to auto manufacturers is cheap labor.

The Center for Automotive Research, a Michigan-based think tank, found that in salaries and benefits, car companies pay an average of $8 an hour for Mexican workers, while in the United States that figure would be four to seven times as high.

“The problem for us is in the salaries,” said Alex Covarrubias Valdenebro, a professor at the College of Sonora, who has studied industry pay levels.

Mexican states, in their competition to attract auto investment, make the deals even sweeter. Companies have been given land, tax breaks and infrastructure by local governments. At the Mazda plant in Guanajuato, the state government agreed to pay half of the employee salaries for six months.

At the Mazda plant, several grievances about the assistant manager coalesced to prompt workers to walk out.

“He would say, ‘You’re stupid. You’re worthless. I don’t know why you have this job,’ ” Capetillo said. “ It was very uncomfortable to work there.”

“He was a despot,” said Jose Luis Rodriguez Rojas, a 23-year-old who worked with air bags.

For a job with 12-hour days, often including weekends, that paid about $75 a week — with $3 of that disappearing into union dues — some decided it was not worth it.

Others, besides the 17 who were forced out, have also left. Maira Guadalupe Corona, 23, quit her job while pregnant because she didn’t think she could care for her baby while working at the plant. She saw how other pregnant women worked the assembly line in physically demanding jobs. Women with infants would complain that the company did not give them sufficient time for breast-feeding.“We were Mazda’s slaves,” said her friend Miriam Vazquez, 24, who also left the plant recently. Although the workers involved in the March work stoppage said they were told at first there would be no repercussions, nearly three weeks later the company demanded their resignations.

“They threatened me. They told me if I didn’t sign, nobody was going to give me work, because they were going to tell all the car companies bad things about me,” Rodriguez said. “Since then, I’ve been looking for work. But I can’t find anything.”

Capetillo said he didn’t want to resign, either . But he eventually agreed to accept a severance. After a couple of months of unemployment, he recently got a job with one of Mazda’s contractors, which does quality control on auto parts inside the same plant. He was using his experience but working for an independent company. He’d been on the job a week and enjoyed it.

But last week, when he showed up at the plant, security guards stopped him. He couldn’t come in, or come back; his name was on a list. In Mazda’s world, he was no longer welcome.

Gabriela Martinez contributed to this report.

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