Border Patrol officials have already warned landowners in Texas that it expects “possible armed civilians” to go onto their properties because of the caravan.
The activists, who are raising money for their plan, are planning to arm themselves with guns and bulletproof vests in an effort to secure America’s border against the caravan of about 4,000 people.
It is dividing opinion among US citizens and fuelling fears of vigilantism.
Some people have posted dire warnings on Facebook about the caravan. One said it was “imperative that we have boots on the ground.” Another wrote: “War! Secure the border now!”
“They’re just laughing in our face,” said Shannon McGauley, president of the Texas Minutemen. “It’s a free-for-all in America.”
Mr McGauley said he already has members at three points of the state’s border with Mexico and expects to add 25 to 100 more people in the coming days.
Another user wrote: “This ‘caravan’ can’t be allowed to bully us and cross the border. A strong border means a safe country.”
I see young, fighting-age men who do not look like they’re starving
Monica Marin, of Oregon, said she had raised about $4,000 (£3120) online to help militias buy supplies. She claimed members of the caravan were dangerous, echoing Mr Trump’s claim that “unknown Middle Easterners” were among the crowd.
“I see young, fighting-age men who do not look like they’re starving. They look like they’re ready to fight,” Ms Marin said.
But in Arivaca, Arizona, residents have posted signs in recent weeks warning that militia members are not welcome.
Marianna Trevino Wright, of South Texas, said she was more fearful of the militias than the caravan.
“It will end badly,” she predicted.
Border watch groups and militias have been patrolling the 2,000-mile southern boundary off and on for more than a decade to tip off Border Patrol about people trying to cross.
President Trump, who has made the issue central to the mid-term elections, has called for the army to be sent to the border and a Pentagon official said the administration will dispatch 800 or more troops.
Although numbers crossing the border have risen this year, they are still far below levels of previous decades.