I already knew that in Spain we called policemen "madera" and "pitufos" due to the color of their uniforms.
Through a very popular song/hymn/chant that recently became very popular through most of Latin American countries, I came to know that in Chile (where this chant is originally from) policemen are referred in a colloquial way as "pacos".
The relevant part of the lyrics of the chant just says:
Son los pacos
Understanding that this did not refer to the Franciscos or the world, I did some research and the Diccionario de Americanismos said "Pacos" are policemen in Chile:
Paco de la esquina. m. Bo, Ch. Policía encargado de la vigilancia de un barrio o manzana. pop + cult → espon.
How did "paco" get to be a colloquial way to refer to police?