Four people have been arrested after eight "slaves" were freed during a raid on a caravan site in Bedfordshire.
Some 65 officers took part in a dawn raid at the Green Acres caravan site near Leighton Buzzard following reports that a number of individuals were being held as "modern slaves".
Eight suspected victims - seven men aged between 20 and 46, and a 17-year-old boy - were found at the plots.
Police say some had been kept at the Gipsy Lane site for "many years".
They are said to be in a "poor state of physical health" and are receiving medical and psychological treatment at an undisclosed location.
Four residents of the plots, two men and two women, were arrested under the Coroners and Justice Act 2009.
Chief Inspector Tania Coulson said: "The men we found at the plot were in a poor state of physical health and the conditions they were living in were cramped and filthy.
"We believe that some of the victims had been living and working there in a state of virtual slavery, some for just a few days/weeks and others for many years.
"The offences being investigated are extremely serious.
"We understand that some people may have information about what has been happening, but have been too scared to come forward and report it.
"People who do want to talk to us about this matter should contact us on 101, or Crimestoppers, which is both anonymous and independent from the police. You do not have to give your name, but if you do we will keep it secret."
Officers involved in the raid were supported by a dog section and police helicopters.
In 2011, 24 "slaves" were freed from the site after a police raid. Four people were later convicted.