Three hundred officers have carried out early morning raids linked to suspected exploitation of migrant workers.
Nine people were arrested in Wisbech and March in Cambridgeshire and King's Lynn in Norfolk.
The operation is the biggest of its kind in the area and involved police, the National Crime Agency and the Gangmasters Licensing Authority (GLA).
It comes two weeks after a BBC investigation into the plight of workers in the Fens.
Eastern Europeans, mostly from Latvia and Lithuania, are employed to pick leeks across the region for UK supermarkets.
The nine arrested - seven men and two women - were taken into custody at about 03:30 BST in simultaneous raids.
Among them is Latvian national Ivars Mezals, who is in his mid-20s and was arrested at a house in Wisbech.
Legal gangmasters Roberto Mac and Martyn Slender, both of March, are also being questioned.
Roberto Mac Ltd and Slender Contracting Ltd have had their licences suspended with immediate effect.
Clothing and food
A reception centre has been set up to accommodate workers displaced by the raids.
Cambridgeshire Police said the Salvation Army would help support potential victims of trafficking.
The Red Cross is offering first aid, clothing and food as well as people for victims to speak to.
The raids were the culmination of months of investigations by police and the GLA into the exploitation of migrant workers in the Wisbech area.
It began when officers looked into the poor living conditions many migrant workers were being housed in.
Chief Inspector Mike Winters said the operation was "targeting those who gain from others' suffering".
"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," he said.
"They are paid wages well below the legal minimum for extremely long hours and their pay is often taken to service debt on their accommodation."
Police said before Tuesday that they had already identified 34 victims, 11 of whom were being given support and protection.
Eight of the nine people arrested are being questioned on suspicion of conspiracy to defraud. They are:
- Three men, all aged 44 and from March
- A 26-year-old man from Wisbech
- Two 27-year-old men from Wisbech
- A 27-year-old woman from Wisbech
- A 36-year-old woman from Wisbech
A 22-year-old woman from Wisbech was arrested on suspicion of possessing a section 5 firearm, namely a pepper spray, and was taken into custody at King's Lynn.