A factory has been raided as part of an investigation into modern day slavery, leading to four arrests.
Concerns were raised about the welfare of staff employed at the premises in Gateshead.
Northumbria Police said the men, aged between 29 and 42, were arrested on suspicion of offences under the Modern Day Slavery Act.
Officers are questioning about 90 employees to establish if they are victims of exploitation.
The arrested men have been released as inquiries continue.
The employees were taken to a reception centre, and the British Red Cross said trained staff were providing them with "emotional support ... and practical necessities".
A police spokesman described it as a "significant operation" and said that inquiries began following a health and safety visit to a connected premises.
He added: "There are individuals out there who are trafficking workers into this country and exploiting them for their own financial benefit and we will work hard to put that to a stop."
The spokesman said the workers at the site were of a number of different nationalities but the majority were Eastern European.