Police raided a house in Lynn during an operation to crack down on the exploitation of migrant workers.
Nine people were arrested and two recruitment agencies had their licenses suspended after police targeted sites in Wisbech, March and Lynn during the early hours of Tuesday.
A total of 300 people took part in Operation Endeavour, which was led by Cambridgeshire Police and the Gangmasters Licensing Agency.
A house in Turbus Road, North Lynn, was raided at 4am. No-one was arrested.
Eight people, seven men and one woman, were arrested on suspicion of conspiracy to defraud.
A 22-year-old woman was arrested in Wisbech on suspicion of possessing pepper spray.
Fenland commander, Chief Insp Mike Waters said: “Victims are promised a better life in the UK with well-paid work but often end up in over-crowded accommodation and immediately placed in debt to a gangmaster who controls their affairs.”