Central American migrants approaching US-Mexico border crossing sites in recent days to apply for asylum are getting a taste of what may be ahead for the human “caravans” still hundreds of miles to the south, as Donald Trump further hardens his immigration policies and rhetoric.

A normal scene in El Paso, Texas, finds US border agents, with handguns in holsters on their belts, routinely supervising migrants crossing a bridge from Mexico towards the port of entry on the US side. But earlier this week, individuals and small groups, including parents and children, found the agents with assault rifles instead, blocking their path and turning them back halfway across the bridge.

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Last Sunday, the border was closed temporarily while Customs and Border Protection (CBP) agents ran drills in riot gear, to prepare for the kind of shutdown the president has threatened if migrant caravans crowd the southern border. And many thousands of active duty US troops are making their way to the border in armored vehicles.

Meanwhile, a man and his son who were stopped at the halfway point of the bridge that connects Ciudad Juárez, Mexico, with El Paso, Texas, spoke to the Guardian, but were too afraid to give their names.

“I have been two days here at the bridge. When we arrived, we were told to wait but they haven’t given us any information of how long we’ll have to wait,” the father said. He and his teenage son fled gang violence in their hometown of Huehuetenango, Guatemala, seven months ago, he said.

The pair sat with a handful of others waiting for information, leaning against the wall of the path approaching the bridge. Volunteers from the lone migrant shelter in Ciudad Juárez, Casa del Migrante, and other samaritans from both sides of the border brought them some sandwiches, blankets and fresh clothes.

Facebook Twitter Pinterest Migrants receive food handouts at the US border Photograph: Edwin Delgado/The Guardian

They may be battling the odds, but as the Trump administration prepares for a fight with migrants, perhaps literally, American aid groups are gearing up, ready to go head to head with the authorities to protect people’s right to have their cases heard.

“Under both international and US law, anyone who claims a fear of persecution in their home country has a right to apply for asylum,” said Robert Painter, director of pro bono services and communications at American Gateways, a Texas not-for-profit organization providing legal services to migrants.

He added: “We expect the Department of Homeland Security to honor that right and ensure that everyone seeking asylum is put through a fair adjudication process.”

Asylum-granting power lies with US immigration judges.

Advocates fear the administration is trying to undercut the only legal avenue for migrants to make their asylum claims, as Trump has made it clear he doesn’t want to hear the migrants’ stories, instead threatening to cut aid to Central America, close the border and deploy as many as 15,000 soldiers, according to the latest report.

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“It’s the complete opposite of a smart response on migration. We see this idea of sending troops to the border as another attempt by the Trump administration to manufacture a crisis where there isn’t one,” said Shaw Drake at the American Civil Liberties Union (ACLU) Border Rights Center in El Paso. “We [already] have ample law enforcement presence at our borders. Migrants are certainly not a threat to anyone, much less a threat that requires troops to be deployed into the backyards of border communities.”

The homeland security secretary, Kirstjen Nielsen, said last week that troops were not intending to shoot migrants “right now”. But advocates are concerned.

“My greatest fear is that US government officials will bring about more violence and suffering on families who have done nothing wrong – only sought safety and freedom in the United States,” said Conchita Cruz, co-founder of Asylum Seekers Advocacy Project.

The Department of Homeland Security did not respond to a request for comment on how federal officials are preparing to handle any large groups of migrants approaching the border.

According to a report in the New York Times, the administration is considering multiple options, including a renewed version of the controversial family separation policy that would make parents entering the US with their children choose between surrendering the minors to foster care or being imprisoned as a family and waiving their children’s right to have detention limited to 20 days.

The government is also weighing further tightening asylum rules, speeding up deportations and extending the use of GPS ankle monitors for those with court dates.

Play Video 1:04 'Nobody's coming in': Trump prepares to send troops to border to stop migrant caravan – video

And it is considering a temporary ban on all migrants from the Central American region entering the US, citing national security, according to the New York Times report, a move that advocacy and migrant rights groups say would be immediately challenged in court.

“This is straight-up a Latino ban,” said Jess Morales Rocketto, chair of the advocacy group Families Belong Together, adding: “The Trump administration and the Republican party have become the party of cruelty to families … This goes against our values as Americans.”

According to CBP data, in fiscal year 2018, about 520,000 undocumented immigrants were apprehended by CBP. The average number of annual arrests from 2000 to 2018 is approximately 741,000, 30% more than this year. The ACLU’s Drake said that DHS now has a “vastly greater” budget and number of immigration officials than at the time when arrests were double or triple the current levels.

In April, another caravan of migrants made their way from Central America all the way to the San Diego and Calexico ports of entry in California. They were mostly allowed to enter and apply for asylum, but after that, Trump cracked down, with the controversial zero-tolerance and family separation policies.

With the midterm election just days away and Trump keen to turn the national conversation away from domestic terrorism and an antisemitic mass shooting, Drake said: “There is no doubt they view this as an opportunity to ramp up and bring back some of the cruelest policies they’ve implemented,” adding: “The US is fully capable of processing and receiving migrants; our reaction will say more about who we are and who we want to be.” The ACLU will resort to court challenges wherever necessary, he said.

Though the current caravan remains far from the border, in recent days more than 70 migrants from Guatemala and Cuba who intend to ask for asylum arrived at the Santa Fe International Bridge, which connects downtown Ciudad Juárez with El Paso.

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The father and son from Guatemala and small clutches of other migrants waited on the walkway. Most who were willing to talk were unaware of the larger caravan making its way from the south. They are aware of hostility from US leadership but say they are driven to make the dangerous trek to the US.

Facebook Twitter Pinterest Aid groups have condemned Trump’s approach to immigration, saying it violates American values. Photograph: Edwin Delgado/The Guardian

“We left because of necessity. In Guatemala, there are no jobs. If I stayed in Guatemala, we would’ve run out of money and not have anything to eat,” said another man traveling with his wife and two children. “I want to come here to work. We’re not bad people.”

A migrant at the bridge from Cuba was fleeing the oppression of the Castro regime, he said.

He felt confident he would pass the “credible fear” asylum test because some of his friends and family are political prisoners in Cuba.

While traversing the Chiapas region of Mexico two weeks ago, he saw the migrant caravan coming from Central America. Instead of joining it he rushed ahead, hoping to be admitted before any possible shutdown of the border, he said.

A spokesman for CBP said the agency was monitoring the caravan.

“We have been making – and will continue to make – necessary preparation. Regardless of the operational contingencies we may face, please know this: we will ensure border security – we will not allow a large group to enter the US unlawfully, we will act in accordance with the highest principles of law enforcement, and we will treat intending migrants humanely and professionally at all times.”

Drake decried Trump’s election tactics. “The US is a beacon of hope and this administration is dragging us to the pit of anger and fear,” he said.