Larry Mitchell Hopkins, 69, of Flora Vista, New Mexico, was arrested Saturday on charges of being a felon in possession of firearms and ammunition, the FBI Albuquerque office said.
Hopkins was arrested in Sunland Park, New Mexico, which lies right on the border with Mexico and is just 8 miles northwest of El Paso, Texas.
"This is a dangerous felon who should not have weapons around children and families," New Mexico Attorney General Hector Balderas said in a statement. "Today's arrest by the FBI indicates clearly that the rule of law should be in the hands of trained law enforcement officials, and not armed vigilantes."
The spokesperson for the group attacked Balderas and New Mexico Gov. Michelle Lujan Grisham for the arrest.
"I am confident that Mr. Hopkins will get though this, will fare well," the militia's spokesperson, Jim Benvie, told El Paso ABC affiliate KVIA, while wearing a red "Trump 2020" hat. "The [New Mexico] AG has declared war on American citizens at the order of the ACLU, instructing the governor, in a sense, to effectively find a reason to remove private citizens from assisting and documenting a crisis on the border. It's really sad that she can't use the resources of the National Guard or even the FBI, if they had to, to help protect the border. Instead, they had to infiltrate and set up our camp, and we're confident about our position with this.
"We're not worried about it. It doesn't change anything," he added.
Hopkins was convicted of impersonating an officer and felony gun possession in 2006, according to The Daily Beast.
He leads a group called The United Constitutional Patriots, which states on its Facebook page that its mission is "to uphold the Constitution of The United States of America."
"We uphold this cause against all enemies both foreign and domestic which shall infringe upon the rights of the citizens given by the Constitution," it says. "We are here to serve in time of need at the local and state level and if necessary for our country."
Benvie regularly uploads videos to his Facebook page showing the group's actions detaining migrants.
The group gained attention on Tuesday when it detained over 200 migrants at gunpoint after they crossed into the U.S. near Sunland Park. The group was held by the militia until U.S. Border Patrol agents responded and took them into custody.
A spokesperson for the group told KVIA that the group never points a gun at migrants and they weren't forced to stay.
The American Civil Liberties Union (ACLU) immediately came out against the action, and blamed it on the rhetoric of the president.
"The Trump administration’s vile racism has emboldened white nationalists and fascists to flagrantly violate the law," the ACLU said in a statement Friday. "This has no place in our state: we cannot allow racist and armed vigilantes to kidnap and detain people seeking asylum."
Mexico’s Foreign Ministry released a statement on behalf of the government expressing "profound concern about the activities of intimidation and extortion of migrants by groups of militias on the New Mexico border."
The group wrote Friday on its Facebook page that Paypal had permanently suspended its fundraising account.
ABC News' Matthew Fuhrman, Ben Jimenez and Josh Hoyos contributed to this report.