The 'commander' of the group, which posted videos showing their members holding migrants, was arrested on a federal complaint charging him with being a felon in possession of firearms and ammunition.
The FBI on Saturday said it had arrested Larry Hopkins, a member of an armed group of US citizens who have been stopping migrants illegally crossing the US-Mexico border in New Mexico.
Hopkins, 69, also known as Johnny Horton, was arrested in Sunland Park, New Mexico, on a federal complaint charging him with being a felon in possession of firearms and ammunition, the Federal Bureau of Investigation said in a statement.
Hopkins had represented himself as the commander of the United Constitutional Patriots (UCP), a volunteer group camped out near Sunland Park since late February. The UCP has posted videos showing their members holding migrants, many of them Central American families with children seeking asylum, until US Border Patrol arrives to arrest them.
The group drew a complaint from the American Civil Liberties Union, and widespread media attention, after an April 16 video showed a group of around 300 migrants surrendering to the UCP at their camp.
New Mexico Governor Michelle Lujan Grisham on Friday said citizens had no authority to arrest or detain anyone and police and justice officials were looking into the UCP's activities.
Hector Balderas, New Mexico's attorney general, described Hopkins as "a dangerous felon who should not have weapons around children and families."
"Today's arrest by the FBI indicates clearly that the rule of law should be in the hands of trained law enforcement officials, not armed vigilantes," Balderas said in a statement.
Hopkins, of Flora Vista, New Mexico, is expected to have an initial appearance in US District Court in Las Cruces, NM, on Monday, April 22, at 10:30 am (1230 ET), the FBI said.