Soldiers in Mexico discovered a total of 652 Central American migrants hiding in three sealed refrigerated cargo trucks, the country's INM immigration agency said on Friday.
The migrants lacked air, food and water in the trailers, a statement said. They are believed to have been victims of human smugglers and have been handed over to the security forces of the state of Tamaulipas.
Among the victims were 349 minors, 198 of them unaccompanied, the agency said.
The vast majority came from Mexico's southern neighbour Guatemala, according to the report.
The trucks had reportedly been stopped Thursday evening at a military checkpoint in the municipality of Hidalgo in north-eastern Mexico, just over 300 kilometres from the US border.
Four people were detained, Tamaulipas security forces announced.
The numbers of people apprehended entering the US illegally at the border with Mexico have risen sharply this year - more than 200,000 in both July and August. Many unaccompanied children are among them.
Most of the migrants are from Mexico or one of the Central American countries of Guatemala, Honduras and El Salvador, where poverty, violence and corruption are rife. Many have also been affected by natural disasters like floods, droughts and hurricanes.
Many people from other crisis-ridden countries in the region, such as Haiti, Cuba and Venezuela, have also recently ventured on the long, dangerous journey through Mexico.
Many migrants do not even reach the US border, but are stopped by Mexican soldiers and sent back.