Twenty-six people were arrested in Belgium and France during an operation caused by the death of 39 Vietnamese migrants in a truck in the United Kingdom in 2019.
A British report found that the migrants all died from suffocation and hypothermia.
Thirteen people, including Moroccans and Vietnamese, were detained in Brussels and 13 others in Paris, according to the authorities.
Belgian prosecutors said the suspects had probably “transported up to several dozen people every day for several months”.
The smuggling ring, they said, was suspected of having made the October 2019 trip possible.
However, Essex police, quoted by the Irish public broadcaster RTÉ, later said that the arrests were not directly related to his investigation.
The European Criminal Justice Cooperation Agency (Eurojust) said police carried out cross-border raids on Tuesday morning in an operation involving four countries – the United Kingdom, France, Belgium and the Ireland – as well as Europol.
A number of people had previously been arrested in connection with the deaths, including several in Vietnam.
Truck driver Maurice Robinson pleaded guilty to 39 counts of manslaughter last month.
At the same hearing in Old Bailey, London, co-accused Gheorghe Nica denied 39 counts of manslaughter. Three other men charged with other death-related crimes also appeared via video link.
A trial on the remaining charges is scheduled to begin on October 5.
The bodies of Vietnamese nationals were discovered in an industrial area shortly after the truck arrived in the United Kingdom on a ferry from Zeebrugge in Belgium.
Among men, women and children, there were 10 adolescents, including two 15-year-old boys.
Pham Thi Tra My, 26, died. She sent a message to her family on October 22 that she could not breathe and that her “trip to a foreign country had failed”.