Immigration officers raided a sandwich factory in east London last week.
Several vans, backed up by unmarked cars, arrived at around 3.30am at Greencore in Tower Hamlets.
The company makes sandwiches which are sold in major supermarkets, such as Asda.
Officers took 32 people into custody, the majority being agency workers.
Several were handcuffed as they were led off.
Workers were photographed and fingerprinted on site.
The workforce is about 300?strong and almost entirely made up of migrants.
A witness told Socialist Worker that the officers divided the workers into eastern Europeans and Africans.
All those detained were black Africans.
Greencore says that it checks the immigration status of all workers it employs, including those hired through agencies.
Witnesses said that officers confiscated company records and used bolt cutters to open people’s lockers.
A witness said workers were terrified by the raid which showed the reality of harassment that migrant workers face.