More than 50 suspected illegal workers have been arrested on coaches heading for food factories in Northamptonshire and Lincolnshire.
They were among more than 130 passengers on three vehicles stopped in Leicester on Thursday morning.
Altogether 120 police and immigration officials were involved in the arrests. Some staff are thought to be Brazilian.
The operation was carried out after the Home Office received information from a recruitment company.
A total of 133 people were stopped after police, in conjunction with immigration officers, intercepted two coaches on the A6 near Oadby and one in Humberstone Road between 0500 and 0600 BST on Thursday.
The action followed a tip-off by a recruitment company in Leicester after staff there voiced concern about a large number of foreign workers suspected of using fake documents.
The Home Office said it has emerged 35 of the workers stopped on Thursday are illegal immigrants from Brazil.
Altogether 78 people have been released and 20 are still being questioned.