More than 400 of the migrants were said to be from Guatemala More than 500 illegal migrants have been discovered crammed into the trailers of two trucks heading from Mexico towards the US.
The 513 migrants, from Latin America and Asia, were found travelling in “inhumane” conditions, police said.
Police made the discovery in the state of Chiapas, close to Mexico’s the border with Guatemala.
Officials say the number of people involved makes it one of the biggest cases of suspected human trafficking.
The trucks were stopped on a highway near the state capital of Chiapas, Tuxtla Gutierrez.
The drivers tried to escape but police arrested them before they could flee.
Police said the migrants, including women and children, were about to be smuggled illegally into the US.
They 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.
“It is the largest ever such operation to rescue illegal migrants travelling in inhumane conditions,” a police spokesman told the AFP news agency.
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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