Twenty-four men suspected of being held against their will have been found during a raid at a travellers' site.
Four men and a woman were arrested on suspicion of committing slavery offences in the raid at Green Acres travellers' site, Leighton Buzzard, Bedfordshire, on Sunday.
The men, who are English, Polish and Romanian, were found in "filthy and cramped" conditions, police said.
Detectives believe some may have been there for up to 15 years.
Those arrested are being held on suspicion of committing offences under the Slavery and Servitude Act 2010.
They are being held at police stations across Bedfordshire and Hertfordshire.
Weapons, drugs and money were also found at the site, police said.
More than 200 officers were involved in the raids.
'Shockingly filthy conditions'
The men, all believed to have been victims of slavery, were taken initially to an undisclosed medical centre for treatment.
They have been placed in the care of the local authority.
Det Ch Insp Sean O'Neil said: "The men we found at the site were in a poor state of physical health and the conditions they were living in were shockingly filthy and cramped.
"We believe that some of them had been living and working there in a state of virtual slavery, some for just a few weeks and others for up to 15 years."
He added that the men appeared to have been "recruited" from soup kitchens and benefits offices and included people with problems such as alcoholism.
He said: "They're recruited and told if you come here we'll pay you £80 a day, we'll look after you give you board and lodgings.
"But when they get here, their hair is cut off them, they're kept in in some cases [in] horseboxes, dog kennels and old caravans, made to work for no money, given very, very small amounts of food.
"That's the worse case. Some are treated a little bit better, but they were told they could not leave and if they did they would be beaten up and attacked.
"But in fact some people did leave and told us what was going on and when we looked back since 2008 we were aware of 28 people who had made similar accusations."
Police have appealed for help and ask anyone with any information to contact them in confidence.
During the operation, Bedfordshire Police were assisted by officers from the UK Human Trafficking Centre.
Police joined forces with dog handling sections, helicopter and firearms support units, to execute search warrants at the traveller site.