The Turkish authorities have freed 160 workers out of almost 600 detained after a protest over work-related deaths and bad conditions at the construction site for Istanbul’s third airport.

Hundreds of workers were arrested on Friday night by security forces busting up a demonstration outside Istanbul Grand Airport, a massive development that the government says will be the biggest in the world, AFP reported.

Another 20 protesters were arrested on Saturday as they gathered to support the detainees.

“Nearly 600 of our friends were arrested, and 160 were freed last night,” Ozgur Karabulut, who heads the Dev Yapi-Is union, told AFP. “We don’t have any information about the others, nor do their lawyers nor the MPs,” he told a press conference in Istanbul.

Out of the approximately 35,000 people employed on the mega-project, which is due to be completed next month, 35 have died on the job, half of them in work-related accidents,  according to Transport Ministry figures.