The migrants – 31 men and eight women – were found dead in the truck in an industrial zone east of London in October, sparking an international outcry.
The driver of the lorry has already admitted manslaughter over the deaths but Tuesday's arrests targeted the ring of smugglers suspected of organising the migrants' journey.
Police swooped in a series of raids around Brussels and Paris as part of a probe also involving British and Irish investigators.
In Belgium – where some of the victims stayed before their fateful journey – police held 13 people, including 11 Vietnamese nationals.
“The network set up by the smugglers is suspected of having likely transported up to several dozen people every day for several months,” Belgian federal prosecutors said in a statement Wednesday.
“The organisation focused on transporting refugees from Asia, particularly from Vietnam.”
Prosecutors suspect the gang organised the transport of the Vietnamese migrants in the container where they died.
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Network operated even under lockdown
Most of those arrested in France are also Vietnamese, according to an investigation source.
The probe has discovered that the migrants who died were loaded onto the truck in northern France, and that the network continued its operations even after the tragedy, charging €15,000 to €20,000 to cross from France to Britain.
Even the coronavirus lockdown did not stop the gang's smuggling activities, the source said.
The tragedy shone a spotlight on the extraordinary dangers migrants are willing to risk to reach Britain, with some paying smugglers up to $40,000 for the perilous journey.
Post-mortem tests found the victims died from lack of oxygen and overheating, and one sent a poignant text message to her family in Vietnam as she lay dying in the truck.
The victims came from impoverished and remote corners of central Vietnam, a hotspot for people willing to embark on dangerous journeys in the hope of striking it rich abroad.
Many are smuggled illegally through Russia or China, often left owing huge sums to their traffickers and ending up working on cannabis farms or in nail salons.
The driver of the lorry, Maurice Robinson of Northern Ireland, last month pleaded guilty to manslaughter over the 39 deaths.
Four other men are on trial in London over the tragedy, while another man, Ronan Hughes, is facing extradition from Ireland to Britain on 39 counts of manslaughter and one of conspiracy to commit unlawful immigration.
Hughes is accused of organising and controlling the drivers in the trafficking operation.