The Polish employees have been paid much lower wages than those set out in the agreement on pay and conditions agreed by unions and management.
The dispute centres on repair work being carried out at a blast furnace plant by a German subcontractor.
Police and representatives of the Finnish Construction Trade Union staged a joint inspection of the Raahe plant on Thursday following a tip-off relating to the low wages being paid to Polish employees.
The Union says it has been told by employees they only receive from three to four euros per hour compared to the agreed minimum wage of just over 15 euros. Employees also claimed they were forced to work eleven-hour shifts seven days a week.
Local union officials added the Polish workers had been ordered to keep quiet about their working conditions.
Some 80 Polish workers are employed at the plant by a German subcontractor.