Three Romanian nationals have been arrested in Preston on suspicion of trafficking people into the United Kingdom for sexual exploitation.
An address in Longworth Street was searched by Lancashire Police officers, who found four women thought to have been brought to the city from Romania.
The men – aged 33, 35 and 42 – are in custody, while the women are being spoken to by specialist officers.
Police said 60 officers and staff had been working on the operation.
Insp Paul McLernon, from Lancashire Police, said: „This area of policing is complex and identifying human trafficking/modern slavery can be difficult.
„In many cases organised crime groups are behind the trafficking of people into the UK and they will target some of the most vulnerable people in our society.”
The force said it wanted to encourage victims of trafficking to come forward.