The dehydrated and hungry migrants were found when the truck was searched at a highway checkpoint, said the immigration official, who spoke on condition of anonymity because he was not authorized to talk on the record.
Police detained the truck's driver and his assistant, both of whom will be transferred to a maximum-security prison, the official said.
The migrants were mainly from Brazil, Guatemala, El Salvador and Nicaragua, but also from India. They were packed so tightly into the truck that they had to remain standing, official said.
The official said the migrants had not eaten in 24 hours and were now being given food and water. They are being held at a Chiapas federal police station awaiting deportation.
Each year, hundreds of thousands of illegal migrants cross Mexico's southern border on their way to the United States. They are often smuggled in brutal conditions, packed tightly inside tractor trailers on long journeys. They are subject to robbery, extortion and kidnapping along the way.
Loa, a 23-year-old from El Salvador, was among the migrants detained Sunday. He was prohibited by immigration authorities from giving his last name.
Visibly haggard, Loa was buying a meat-filled sandwich passed to him through prison bars from a street vendor.
"It was very hot and we had no water," he told The Associated Press about his trek, which began Friday. His plan had been to reach Los Angeles.
The United Nations estimates that smuggling migrants into the United States is a $6.6 billion business annually. That doesn't include another $1 billion paid by thousands of non-Mexicans to cross from Guatemala into Mexico and then travel north toward the U.S. border, according to a 2010 U.N. report on transnational crime.
In May, 513 people were apprehended in two trailers in Chiapas, bordering Guatemala. They represented a cargo worth at least $3.5 million. Another trailer filled with 219 people was discovered in January.
William, 43, one of the 210 migrants picked up Sunday, said he paid $3,000 to smugglers to truck him from Guatemala to the United States to work. He knew that once he was caught there would be no refund.
"There's no work back home," said William, who would not reveal his last name.
The truck was traveling on a highway bordering Veracruz state when it was stopped by authorities.