Eleven men who were thought to be victims of human trafficking have been rescued, Derbyshire Police said.
Houses in Derby's Sunnyhill Avenue, Balfour Road, Violet Street and Cameron Road were raided on Monday.
Seven people - four women and three men - were arrested on suspicion of conspiracy to commit human trafficking and money laundering.
The victims are thought to have been brought into the UK from eastern Europe for "labour exploitation", police said.
The men have been taken to a special reception centre and are being cared for by the county and city councils, the Salvation Army and the Red Cross.
Two of the women arrested - aged 22 and 37 - and the two men were also arrested on suspicion of fraud.
Det Ch Insp Paul Callum said: "We are at the early stages of this investigation and we are working with several agencies as part of the inquiry and to ensure the safety and wellbeing of the men we believe to be victims.
"Human trafficking is a serious offence. In many cases, people are brought into the UK and are promised the chance of a good job and a better life.
"But this often turns into exploitation where victims are made to work with little or no pay and without money and the correct documentation, it can be very difficult for them to escape."