The criminals pushed migrants into the back of large lorries while drivers were sleeping, said Europol.
Police say they have dismantled a migrant smuggling ring transporting Afghans to western Europe.
The network is thought to have smuggled around 200 migrants from Romania to France and Germany, according to Europol.
The criminals pushed migrants into the back of large lorries while drivers were sleeping, it said.
"Migrants were paying between €1,000 and €2,000 for the dangerous trip," the authority said.
"Hidden between goods such as tree trunks and pallets, the migrants endured low temperatures and inhuman conditions during several days of transport.
"According to the investigations, the truck drivers were not aware that they had people in the back of their trucks."
A total of 21 people were arrested in raids on Tuesday (October 5), including 16 Afghans, four Romanians, and one Iranian national. Six more suspects had been identified.
Police also seized cash, dozens of mobile phones, other electronic equipment, and ten vials of morphine.
Officers had carried out searches at a number of properties in Romania, in Timisoara, Timiş county, and the capital city, Bucharest.
"The German Federal Police and the Romanian Police ... dismantled an organised crime group involved in the migrant smuggling of predominantly Afghan nationals," Europol said in a statement.
The group had been smuggling Afghan citizens from "refugee camps and other places where asylum seekers gather".
Investigators said the criminal gang had been operating in the Romanian counties of Timiş and Arad since May 2020.