After hearing a noise, the truck driver and a security guard opened the van's back doors and saw about 15 people of "North African appearance" inside the car, BFMTV broadcaster reported.
According to the media, the migrants escaped as soon as they were found.
The driver was detained and taken to a police station. He said that the van's trailer was closed with a padlock, and there were no signs of damage to the trailer. According to the broadcaster, the van left Luxembourg, made a 24-hour stop in Reims and stopped at the Zenith Paris arena on Friday morning.
An investigation into the incident was launched, the media said.