The leader of an armed group accused of illegally detaining migrants at U.S. southern border... was detained himself by the FBI... and is set to appear in U.S. District Court on Monday.

(SOUNDBITE) LARRY HOPKINS, SAYING: "This country was built on Three things: God, guns, and guts." Sixty-nine-year-old Larry Hopkins was arrested in New Mexico on Saturday, on a federal complaint charging him with being a felon in possession of firearms and ammunition.

His arrest comes after the ACLU called on authorities to put a stop to the militia group's operations.

The United Constitutional Patriots, or UCP, describes itself as a ‘patriot group’ helping U.S. Border Patrol deal with a surge in undocumented migrants.

A spokesman said its members — who dress in camouflage and carry rifles — have helped agents detain over 5,600 migrants in the last two months.

But critics accuse the group of impersonating law enforcement, as well as promoting hate and violence.

Tensions at the U.S.-Mexico border have risen as President Donald Trump continues to push his hardline stance on immigration.

U.S. Customs and Border Protection said in a statement it did not support citizens taking law enforcement into their own hands.

It, instead, encouraged the public to be its eyes and ears on the border.