The LAPD has opened an investigation into the horrifying attack in front of a Hollywood nightclub Wednesday morning ... in an effort to identify the man who K.O.'d an unassuming clubgoer.



TMZ captured the incident on tape outside the Roxbury nightclub  -- when an unknown man took a running start and delivered a ferocious punch to the head of a man who had just walked out of the club.

The victim -- said to be in his 30s -- instantly crumbled to the ground and laid unconscious for ten minutes before an ambulance arrived to the scene.

The victim was rushed to a nearby hospital where he was treated for injuries. No word on his current condition.