Ten potential "modern slaves" have been found during raids on five properties, police have said.
Six people were arrested following the execution of warrants by police as part of a modern slavery operation in Derby.
Three raids were carried out in Rutland Street, one in St Giles Road and one in Balaclava Road.
Three men, aged 32, 41 and 53, and three women, aged 31, 35 and 54, were arrested on suspicion of human trafficking offences.
'Very little pay'
The potential victims, who police believe have been trafficked into the UK from Latvia, are being given support.
Derbyshire Police said the 10 men were probably working "for very little pay" and would not have had access to bank accounts that were set up in their names.
Det Ch Insp Rick Alton said: "We have been working very closely with Latvian authorities on this case and we're grateful for their support.
"We have also been supported by partner agencies including the Gangmasters and Labour Abuse Authority, Derby City Council and revenue officials.
"We are in the early stages of this investigation and the safety and well-being of the men we believe to be victims remains our priority."
He said in many cases, victims were "made to work with little or no pay and without money and the correct documentation, so it can be very difficult for them to leave".
A total of 140 "modern slaves" were found by Leicestershire, Nottinghamshire, Derbyshire and Lincolnshire police forces between January 2014 and the end of 2016, according to a recent FOI request.