MEXICO CITY and LONDON -- Hundreds of migrants from Central and South America were found "crowded and dehydrated" in the back of a tractor-trailer in southeastern Mexico on Thursday, authorities said.

The discovery was made at dawn after authorities stopped the driver of the trailer at a toll booth in between the cities of Córdoba and Coatzacoalcos in the eastern state of Veracruz, according to a press release from Mexico's National Institute of Migration (INM).

A gamma-ray device detected the presence of people inside the container and authorities heard screams and knocking at the back of the unit. When they opened it, authorities found 350 migrants, including men, women and children, the INM said.

A vast majority of the migrants -- 340 of them -- were from Guatemala while six were from Ecuador, three from Honduras and one from El Salvador, according to the INM.

Those who were minors or traveling as a family were placed under the guardianship of the state's System for the Integral Development of the Family. The other adults were taken to the INM's headquarters in Veracruz to begin the process of defining their immigration status in Mexico, INM said.

Meanwhile, the trailer and the driver were taken into custody by the state attorney general's office, according to the INM.