Image copyright Getty Images Image caption Some 600 migrants were found hidden in two trailers
Rising poverty in Central America has led more migrants to risk crossing the US border in search of work.
To find work Mexicans pay a fee to agents who sneak them into the US, to work on farms or in construction, Reuters news agency said.
The workers pay smugglers to bus them from Mexico to the US, a long-distance journey that can take two to three days.
Mexican authorities said they had found 600 migrants in two vehicles in Veracruz state.
Most of them were dehydrated, dehydrated, barely alive and in bad condition, a official said.
Monica Galan, a migration official, told Reuters that many people were being taken to a shelter where they could receive medical attention.
Small buses are run to drop migrants off at checkpoints along the route from southern Mexico to border cities like Nuevo Laredo, where migrants will be registered for legal entry, Reuters reported.
To qualify to work legally they must meet a few basic requirements.
Mexican workers find Mexican workers who are also willing to pay smuggling fees to cross the border legally to reach the US. Mexico has tightened border security along the border in recent years, but migrant workers still manage to sneak in.
The Mexican government has often stepped in to rescue migrants to prevent them from trying to cross without papers, but because the journey is long and dangerous, many Mexicans still send money back home.
Images from Veracruz showed the migrants receiving food and water after they were rescued.
Image copyright Getty Images Image caption Piles of medical supplies lined the side of the road
‘He got down on his hands and knees’
Sergio Alonso Diaz, 28, who is from Sinaloa state in western Mexico, told Reuters he and a group of around 200 migrants had taken the long, winding route in vans because “they wanted to try it for their lives”.
His fellow migrants told Mr Alonso that they paid a $2,000 fee (about £1,600) to smugglers who drove them over the Rio Grande in Texas.
Then on Thursday morning, they started to cross the Veracruz state border with Mexico at the Usumacinta river, according to Reuters.
“We were taking about two hours to cross, but when we arrived [at the crossing], more people arrived and we were intercepted by the police,” Mr Alonso told Reuters.
Images from the scene in that region of Veracruz show migrants being taken out of the truck by police and loaded onto the back of ambulances.
There were also medical facilities on site with fresh water and meals, and portable toilets were laid out, Reuters reported.
“People asked me if I got down on my hands and knees, I tried to figure it out,” Mr Alonso said.
Image copyright Getty Images Image caption Authorities have ordered the release of food and water to migrants after the discovery of the migrants
“They called us to surrender, to give ourselves up, and we turned ourselves in to them… They gave us some water and food.”
Cesar Moreno, an immigration official, told Reuters that the migrants were all Mexican citizens and had entered the country illegally.
The migrants, however, told other media that they had paid to get through.
They said that many had paid as much as $10,000 to smugglers to make the trip, Reuters reported.