Julio Cesar Chavez Jr. was arrested for illegal possession of an assault rifle this weekend in L.A. ... after cops were called to the scene by a concerned fan who believed the boxer may be a danger to himself or others.
TMZ Sports has learned the 37-year-old former champ, son of the legendary Mexican pugilist Julio Cesar Chavez Sr., was taken into custody by LAPD around 1:30 PM ... after Jr. permitted responding officers to enter his home, where they located the illegal rifle.
Law enforcement sources tell us the weapon was an untraceable "ghost gun.