Rights group Amnesty International has accused Qatar of using forced labour at a flagship World Cup 2022 stadium.
Amnesty says workers at Khalifa International Stadium are forced to live in squalid accommodation, pay huge recruitment fees and have had wages withheld and passports confiscated.
It also accuses Fifa of "failing almost completely" to stop the tournament being "built on human rights abuses".
Qatar said it was "concerned" by the allegations and would investigate.
The government said the welfare of migrant workers was a "top priority" and insisted it was committed to systematic reform of Qatar's labour laws.
Last year the country pledged to makes changes to its "kafala" sponsorship system, under which migrant workers cannot change jobs or leave the country without their employer's permission.
But Amnesty warned the proposed reforms would make little difference and said some of the workers were enduring a "living nightmare".
"All workers want are their rights: to be paid on time, leave the country if need be and be treated with dignity and respect," said general secretary Salil Shetty.
Amnesty interviewed 231 mostly South Asian migrants - 132 working at the stadium and 99 on green spaces in the surrounding Aspire sports complex.
It said staff of one labour supply company used the threat of penalties to exact work from some migrants such as withholding pay, handing workers over to the police or stopping them from leaving Qatar.
This amounted to forced labour under international law, Amnesty said.
The Qatari government said its Ministry of Administrative Development, Labour and Social Affairs would investigate the contractors named in the report.
One metal worker from India who worked on the Khalifa stadium refurbishment told Amnesty he was threatened by his employer when he complained about not being paid for several months.
"He just shouted abuse at me and said that if I complained again I'd never leave the country," the worker said.
"Ever since I have been careful not to complain about my salary or anything else. Of course, if I could I would change jobs or leave Qatar."
Another metal worker from Nepal said his life was "like a prison".
Some of the Nepali workers told Amnesty they were not allowed to visit their families after the earthquake last April that killed thousands and left millions displaced.
Shetty also had harsh words for Fifa, accusing it of "indifference" to the abuse of migrant workers, which he said was a "stain on the conscience of world football".
The Khalifa Stadium is part of the Aspire Zone sports complex, where facilities have already been used by some of the world's biggest football clubs, including Bayern Munich, Everton and Paris Saint-Germain, which trained there in the winter.
Amnesty has called on big World Cup sponsors such as Adidas, Coca Cola and McDonald's to put pressure on Fifa to tackle the issue.
"It is time for football's leaders to speak out or be tainted by association," Shetty said.
It wants Fifa to get Qatar to publish a comprehensive reform plan before construction on World Cup projects peaks in 2017.
It also wants Fifa to carry out its own inspections of labour conditions in the Gulf state.
The numbers working on World Cup sites are set to rise tenfold to around 36,000 in the next two years, Amnesty said.
Fifa acknowledged it had a responsibility to ensure human rights were respected in the build-up to a tournament, but also said it was not responsible for "solving all the societal problems" in a World Cup host country.