X-ray equipment detected the immigrants at checkpoints in the southern state of Chiapas, the state attorney general's office said. The drivers of both trucks sped away after authorities ordered them to stop, but police apprehended them nearby.

Inside the tractor-trailers, they found 513 people from El Salvador, Ecuador, China, Japan, Guatemala, India, Nepal, Honduras and the Dominican Republic.

Each of them had paid $7,000 to travel to the United States, the attorney general said.

They had been traveling in "inhumane conditions," Mexico's National Migration Institute said.

Officials said the group was receiving food, shelter and medical assistance while Mexican federal immigration authorities reached out to consulates and embassies to arrange their return to their home countries.

The southern border state of Chiapas, which borders Guatemala, is a major crossing point for Central American immigrants heading north.

In 2010, Mexican authorities apprehended about 25,000 illegal immigrants there, according to statistics from Mexico's National Migration Institute.