Six people have been arrested on suspicion of human trafficking and modern slavery, Derby police said.
Officers swooped in dawn raids on 12 September at four addresses in the East Midlands city, taking three men and three women into custody.
The men, aged 32, 41 and 53 and the women, aged 31, 35 and 54, were arrested on suspicion of human trafficking offences.
Police said the gang smuggled nine "potential" male victims from Latvia, who are currently being supported by Derby City Council and other agencies.
The Latvians had been working for "very little pay" and had no access to bank accounts, added police.
Detective chief inspector Rick Alton said: "We have been working very closely with Latvian authorities on this case and we're grateful for their support. We are in the early stages of this investigation and the safety and wellbeing of the men we believe to be victims remains our priority.
"As a force, we take allegations of trafficking and modern slavery very seriously. 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 leave."
Last month Derby police also arrested three people in the city and rescued several alleged human trafficking victims.
Tens of thousands of victims
Kate Roberts, head of office at the Human Trafficking Foundation, said: "It's always a success for the police to make arrests and rescue alleged victims which has happened in Derby, and we're pleased with that news. Every arrest is a breakthrough."
Modern slavery and human trafficking is "far more prevalent than previously thought", the National Crime Agency (NCA) said in August.
Britain's anti-organised crime agency said there were more than 300 current police operations active in the UK, with cases affecting "every large town and city in the country".
It estimated there were tens of thousands of victims, with some as young as 12 being sold to families in the UK from Europe.
The most common nationalities of victims brought into the UK are people from Eastern Europe, Vietnam and Nigeria.
Will Kerr, NCA director of vulnerabilities, said: "[The abolition of] modern slavery has rightly been made a priority across law enforcement, but it is a hidden crime so the onus is on us to seek it out.
"The more that we look for modern slavery the more we find the evidence of the widespread abuse of vulnerable [people]."
The NCA said the growth in modern slavery is driven by international gangs who recognise the amount of money to be made by controlling people within a huge range of economic activity, rather than just dealing drugs.
Kerr said there were "lots of different outlets" for people trafficked into the UK to be working illegally and against their will.
Examples include those working at car washes and in industries such construction, agriculture and food processing.
They often receive very little pay and are forced to put up with poor living conditions.
Others sold into slavery could be kept in pop-up brothels – where sex workers promised a better life are instead left penniless – while some work in cannabis factories.
Traffickers use the internet to lure their victims with hollow promises of jobs, education and even love, said the NCA.