Mexico has detained nearly 400 Central American migrants who were travelling through the south of the country trying to reach the US border, officials say.
Monday's operation in Chiapas state targeted a group of some 3,000 migrants which included small children.
It is said to be the largest single raid on people travelling in so-called caravans. Officials say those detained had refused to apply for visas.
Mexico is under pressure from the US to stem the flow of people heading north.
There has been a huge increase in men, women and children fleeing El Salvador, Honduras and Guatemala, the three countries where most of those seeking asylum on the US-Mexico border come from.
President Donald Trump again threatened to close the border if Mexico did not do more to stop the undocumented migrants.
A very big Caravan of over 20,000 people started up through Mexico. It has been reduced in size by Mexico but is still coming. Mexico must apprehend the remainder or we will be forced to close that section of the Border & call up the Military. The Coyotes & Cartels have weapons!— Donald J. Trump (@realDonaldTrump) April 24, 2019
Mexican immigration authorities targeted people at the start and end of the group on a road near the town of Pijijiapan, forcing migrants into police vehicles as children cried.
The migrants who managed to escape hid in hills and took refuge at nearby shelters and churches.
"They waited until we were resting and fell upon us, grabbing children and women," Arturo Hernández, a 59-year-old farmer from Comayagua, Honduras, who fled with his grandson, told AP news agency.
"There are people still lost up in the woods. The woods are very dangerous."
Those detained were sent to an immigration station in the nearby city of Tapachula, but it was not clear whether they would be deported.
Also speaking to AP, Denis Aguilar, a factory union leader from San Pedro Sula, Honduras, said this was a "planned ambush" to break up the caravan.
"They grabbed the children... the strollers are abandoned there."
President Andrés Manuel López Obrador said his government was not giving migrants "free passage" not just out of "legal concerns but for questions of safety".
The authorities, he added, were trying to break up the work of human traffickers who are allegedly charging migrants to take them to the US border.
Interior Secretary Olga Sánchez Cordero said the detained migrants, most of them from Honduras, had refused to register for a visa.
What is the situation?
Many of the migrants say they are fleeing persecution, violence and poverty in their countries. Since last year, they have been travelling in large groups, often in their thousands, towards the US border.
They see it as a safer and cheaper alternative to paying large sums to people smugglers, known as "coyotes".
But many small Mexican towns in their route, once receptive, have become hostile to them as the caravans are seen as a burden to government resources and often a source of trouble.
Amid a surge in the numbers of migrants arriving in Mexico, Mr López Obrador's government has limited the number of people who are given humanitarian visas and tried to contain them in the south of country, sometimes in overcrowded immigration facilities.
Additionally, the number of deportations has also increased. Official data says more than 15,000 people were returned in the last 30 days.
Meanwhile, the US government has limited the number of people allowed to apply for asylum each day and many are being forced to wait on the Mexican side of the border.
Others are being returned to Mexico as they wait for their cases to be processed.