The alleged slaves, many of them British, were forced to carry out manual work and threatened with violence if they fled. They were locked up in filthy caravans and horse boxes at the Green Acres caravan site in Leighton Buzzard, Beds.
On Monday night it was reported that more than 100 people were believed to have been held prisoner across the country as part of a slavery network targeting homeless and vulnerable men.
It emerged that raids has also been carried out at four other travellers' sites.
The police investigation has been challenged after nine of the 24 alleged slaves refused to help officers with their inquiries, seven of whom were British.
One of the nine men, who did not want to be named, said he had been living in a caravan on a traveller plot for several years, working as a paver and was being paid £50 a day. He said: "The police told me I couldn't come back but I told them it was my home and if I wanted to go back I would go back."
Police claimed the suspects lured vulnerable men from dole queues and homeless shelters to work at the camp, but people on the site said they came voluntarily because they knew travellers would give them work.
Police said other victims from the site had already been taken to Scandinavia.
Eight British men aged from 30 to 57 were among the victims helping detectives.
Three Polish men, a Latvian and a Lithuanian were also among the group due to be interviewed, as well as two whose nationality is not yet known. The youngest victim found on the site was a 17–year–old British boy who has since been returned to his family.
Four men, all from the same family, were charged last night with slavery offences. Tommy Connors, 30, Patrick Connors, 19, James (Big Jim) Connors, 34, and James (Jimmy) Connors, 23, all of Green Acres caravan site, were due to appear at Luton magistrates' court today.
A fifth person who was arrested, a heavily pregnant woman, was released earlier on bail and will be questioned further after the birth of her child.