A Turkish court on Wednesday jailed 24 workers, including four union leaders, pending formal charges for their alleged roles in a protest at Istanbul’s new $13 billion airport.

The case has cast a harsh light on the country’s bleak labor conditions and threatens to mar President Recep Tayyip Erdogan’s latest prestige project: one of the world’s biggest airports that will serve some 90 million passengers. The airport is set to open Oct. 29, coinciding with the 95th anniversary of the founding of the Turkish Republic.

The workers’ complaints, including dozens of deaths due to an unsafe working environment, underscore why Turkey ranks among the world’s worst countries to be a worker.

The court released 19 people but they are subject to judicial controls ahead of a prosecutor’s indictment, the builders’ union Insaat-Is, said on Twitter.

Security officers raided the airport’s construction site and detained 400 people after workers demonstrated on Sept. 14 over a shuttle bus crash that injured 17 employees. Most of the 400 people who were detained have been released, Istanbul’s governor said.

Conditions have worsened in recent months as contractors race to finish the facility in time for its slated opening, with workers spending up to 70 days on the job without a break, according to an anonymous worker interviewed by online broadcaster Medyascope.tv.

The worker described “countless deaths” at the site, as well as ambulances not turning on their sirens so as to conceal when they collected injured or dead workers following accidents. “They hide it,” he said.

The Transportation Ministry said in February that 27 workers, out of a total 36,000 employees, had lost their lives since construction began in 2014. The ministry denied a report by Cumhuriyet newspaper that quoted unnamed laborers who estimated that 400 people had died and that 500 or more workplace accidents occurred each week.

Work was halted during the protest against unpaid or delayed wages, shoddy safety standards and filthy housing at the onsite camp where many of the workers live. Footage showed workers setting fire to their uniforms and helmets, and gendarmes setting off tear gas and carting off protesters as armored riot control vehicles prowled the site.

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