More than 400 of the migrants were said to be from Guatemala Four people accused of smuggling the migrants were arrested, officials said.
The discovery, made in Mexico's southern Chiapas state, is one of the biggest cases of suspected human trafficking, police say.
The migrants were discovered using X-ray equipment at a checkpoint, about to be smuggled illegally into the US, police said.
They travelled for hours clinging to nets strung inside containers, according to the AP news agency.
Though there were holes for air punched in the floors of the containers, the migrants lacked air and water, they told AP.
"We were suffering, it was very hot and we were clinging to the ropes," one Honduran migrant said.
Criminal gangs Police believe the foreigners had to pay $7,000 (£4,300) per person to the alleged traffickers to take them into the US.
While most of those on board were thought to be from Guatemala, there were others from Ecuador and El Salvador as well as smaller numbers from as far afield as China, Japan, Nepal and India.
There were 32 women and 4 children on board, authorities said.
"It is the largest ever such operation to rescue illegal migrants travelling in inhumane conditions," a police spokesman told the AFP news agency.
The trucks were stopped on a highway near the state capital of Chiapas, Tuxtla Gutierrez.
Around half a million illegal migrants try to cross Mexico each year in a bid to reach the United States.
Human rights groups say many fall into the hands of organised criminal gangs who exploit and abuse them.
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