Hundreds of labourers were arrested late on Friday as the security forces sought to break up a demonstration outside Istanbul Grand Airport, a huge development which Ankara has claimed will be the biggest in the world.
Another 20 were arrested on Saturday as they rallied in support of the detainees.
Some 35,000 people are employed on the project which is due to be completed next month but which has come under fire over its safety record after 27 workers died at the site, half of them in work-related accidents, transport ministry figures show.
But around a quarter of them have now been released, union figures told AFP on Sunday.
"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, which was also attended by other union reps.
At Friday's demonstration, the protesters complained about work-related deaths and accidents at the site, as well as poor living conditions, which reportedly included fleas and bedbugs.
In a statement issued on Friday, airport construction and operating company IGA said management had met the workers and pledged to resolve the issues "as quickly as possible".
Contacted by AFP on Saturday, the IGA made no comment on the arrests.
Despite the figures of 27 on-site deaths released by the transport ministry in April, several workers told AFP the number was likely much higher, saying accidents were very common at the site.
Earlier this year, Turkish President Recep Tayyip Erdogan said the new airport would be the biggest in the world with a first phase capacity of 90 million passengers a year, reaching 150 million by 2023.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)