Part Mass, part-outdoor concert, the ceremonies began with a medley of Cuban folk songs from around Santiago before the pontiff entered the crowd in his Popemobile.
The duo began writing songs when their bands were on the bill of a big rock concert a few years ago, and eventually recorded this album, also called Fiction Family.
Fans who began arriving for the sold-out concert when the doors opened at 18:30 GMT said on Twitter there were boos from some people in the crowd almost an hour before he emerged.
Iron Maiden may have been formed in a London pub the 1970s, but it wasn't until 1985 when they took to the stage at the original "Rock in Rio" concert that their South American fan base began to swell.