An acrobat fell 100 feet to his death during a performance before Green Day took the stage at a music fest in Spain ... and the fatal moment was caught on video.

The acrobat -- Pedro Aunion Monroy from Brighton, UK -- was high above the stage in an illuminated cage during a tribute to Prince when something went terribly wrong and he fell out of it. The performance was in between sets of Alt-J and Green Day at the Mad Cool Festival in Madrid, Spain.

The disturbing footage shows some of the 35,000 people in the crowd scream as they see Monroy free-falling to the ground.

Green Day is taking heat for playing their gig less than 30 minutes after the tragedy, but the band issued a statement saying they weren't informed of Monroy's accident until they walked off stage.