The Trump administration's new plan to accelerate deportation of undocumented migrants has created a sense of deep disquiet in towns such as this one, where families have sent generations of migrants to the United States and are unsure what may happen if they are forced to return.

Esmeralda Sanchez has three daughters living undocumented in the United States. "They don't know what to do, they don't have a plan for if they are deported," she said. "My roof isn't big, but I will have to make a plan – I will have to make a place for them."

Some of her grandchildren are U.S.-born and citizens, while others are undocumented, and the family is uncertain about what would happen to them in the event the parents are deported. "It's terrible to have families cut in half," said Ms. Sanchez, 58, who lives on what she earns selling shoes and lingerie and on cash her daughters send home.

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On Tuesday, the U.S. government released a memorandum detailing steps to implement the Trump executive order on unauthorized migrants; it says that staff and detention facilities will be expanded rapidly to accelerate the removal of those detained and the categories of those who would be targeted for removal will be broadened.

The majority of undocumented immigrants in the United States are Mexicans, about 5.8 million people. They sent $26.97-billion (U.S.) in remittances home to Mexico last year – 2.6 per cent of GDP, according to the Centre for Latin American Monetary Studies.

"The new Trump plan … puts Mexico in a difficult position because we're not prepared for a massive return," said Guadalupe Chipole, who runs the Centre for Care and Support of Migrants in Mexico City.

Many of those who now face deportation have been living in the United States for years; they may have no homes here, and only fragile ties with the families and communities they left behind and could not visit through that time, she said. In many places, the conditions that drove them to leave have not changed.

"Most of these people left hoping for an easier life, for ways to earn a living, access to services … and now they are going to come back to these same places with a lack of jobs, as well as a growing problem of violence and the presence of organized crime."

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"Clearly, this is going to cause more social pressure," she added.

"We don't know how many people will be coming back," Juan Carlos Romero, a federal senator, said in an interview. "We don't know what we're going to be facing. We're not ready for them."

On Wednesday morning, Mexican Foreign Minister Luis Videgaray reacted angrily to the Trump plan and made one of his government's strongest statements to date about its commitment to protecting migrant rights.

"The Government of Mexico will act by all means legally possible to defend the human rights of Mexicans abroad, particularly in the United States," he said. The government "will go to international organizations beginning with the United Nations to defend, in accordance with international law, human rights, freedoms and due process for Mexicans abroad."

Gretchen Kuhner, director of the Mexico City-based Institute for Women in Migration, said migrants in the United States are feeling "terror and ambiguity" in the wake of the Trump announcement. "Our concern is that people are going to be picked up and their kids are going to be left behind," she said. Her organization is urging undocumented Mexican women in the United States to leave an emergency plan with schools, in case parents do not show up to pick up their children one day, and to make sure a trusted friend or relative has a temporary custody letter.

The institute is already working with dozens of women deported by the Obama administration to help them get their children to Mexico and to try to restore parental rights that may have been terminated by child protective services.

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Ms. Kuhner said she anticipates that most of those deported will go first to the towns and villages where they have family that will take them in, but before long will need to move to cities in search of work. And many will head for border towns, she said, where shelters are already overflowing – because if they have U.S.-citizen children, they can be brought across the border for visits by documented relatives.

The institute and other advocacy organizations recently succeeded in persuading the government to change education policy so that Mexican schools are required to accept all children, regardless of their citizenship or immigration status. But the school system remains ill-prepared to handle children who arrive with limited knowledge of Spanish and are used to a very different curriculum, she said.

Socorro Prado, a 53-year-old tailor in San Luis de la Paz, described her family's anxiety for her brother, who is in the United States with his three children, only one of whom is U.S.-born. "My brother owns a piece of land here, but he doesn't have a house. And who knows what kind of work he'd be able to do here if they send him back? Any job you can get here you need a car. He has a car there, but he won't be able to bring it if he's deported. And nobody here is in a position to give him a car."

The municipality of San Luis de la Paz, which is home to 121,000 people, received $104-million in remittances last year, a significant chunk of its economy. "If they deport lots of people, there will be a crisis of unemployment," said Rodolfo Nunez, a 43-year-old maintenance worker who himself spent seven years working undocumented in California. "There are no jobs now as it is."

The people in town with successful businesses depend on capital sent by relatives in the United States to set them up, he said, and they depend on customers who are also living on remittances. "If they start deporting everyone, that will be the end of all these things," he said, gesturing at the appliance and clothing shops that ring the town square.

In Mexican towns where the economic prospects remain limited and where there is a long tradition of migration to the United States, there is as yet no sign that the Trump administration's orders are deterring would-be migrants.

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In Xichu, for example, which sits at the end of a road winding into the sierra above San Luis de la Paz, a group of young men left a couple of weeks ago. They crossed into Texas undetected, the town's economic development officer Manuel Casas said, while another batch went this past week. "We're waiting to hear about them."

It is difficult to assess, he said, whether the Trump orders will change the flow of migrants who attempt to cross, or what accelerated deportations will mean for the town. "Ninety per cent of people here have either gone to the U.S. or live off someone there," Mr. Casas said.

And while people who live close to the industrial centres may find jobs at home in Mexico, in more isolated towns such as his, "Your only option is still to leave."