Four people have been arrested by a team of 100 police officers conducting an inquiry into slavery and servitude.
An Eastern European man has been taken to hospital after the raids in Cardiff, Monmouthshire and Marshfield.
The inquiry was sparked by the discovery of another man found living in poor conditions in Marshfield earlier this year.
Officers have also said they are investigating if a body has been buried at a farm in Marshfield.
The four arrested - a 66-year-old man and a 42 year old both from the Marshfield area, a 36-year-old man from the Cardiff area and a 42-year-old woman from Penhow, Monmouthshire - are being questioned.
A man was taken from the Marshfield site for hospital treatment where his physical and mental condition is being assessed.
His age is unknown but officers confirmed he is of Eastern European origin.
The investigation - called Operation Imperial - has centred around evidence and intelligence suggesting that people were being kept in poor conditions at the Marshfield site and forced to work for no pay.
Earlier this year, 43-year-old Darrell Simester from Kidderminster, Worcestershire, was found living at the site having been missing for over a decade.
Det Supt Paul Griffiths of Gwent Police, who is leading the investigation, said: "Today's action is the latest phase of a long-running investigation into alleged offences of slavery and servitude.
"The investigation began when a 43-year-old man was found to be living at the Marshfield site in very poor conditions having been reported missing by his family for 13 years," he said.
"Since then a specialist team of detectives has been investigating and gathering intelligence to ascertain whether this part of a larger criminal conspiracy.
"In addition to the suspected offences in relation to slavery and servitude, we are also acting on intelligence which suggests that the body of an unknown person may be buried at the Marshfield site.
"As such specially trained officers, equipment and a forensic archaeologist have been deployed to investigate this.
"Because of the scale of the investigation we are fully aware the operation has caused disruption to other residents, especially in the Marshfield area, and we thank them for their continued co-operation and understanding."
Gwent Police is working in conjunction with the Serious Organised Crime Agency (SOCA), other forces, the UK Human Trafficking Centre, RSPCA and the Red Cross.