This went down Saturday at McCarran Airport, where an Alaska Airlines flight that was set to take off for Portland got an unwelcome visitor that prevented it from taking off. It was this dude, who's said to have come out of nowhere -- but somehow made his way onto the wing.

People were amused at first, snapping photos of the guy as he sat out there awkwardly. Eventually cops were called and tried intercepting the guy -- but it was tricky, as officers had to scale the plane themselves and get out there on the wing with him.

The man did not want to go quietly, as you can see ... he avoided the cops and went out even further on the wing tip. Then, he removed his socks and shoes -- tossing them on the runway -- and proceeded to try and clip what little wing there was left to grab.

Long story short ... it didn't end well for this fella. He lost his grip and then tumbled down to the ground, where he hit his head first and then fell limp like a rag doll. Cops didn't hestitate though -- they pounced on him and threw on the cuffs. Reports say he was arrested.

The Las Vegas Metro Police Department also says they believe the guy hopped over a perimeter fence at McCarran and ran rampant onto the grounds. They said he might have mental issues too.