Jason London crapped himself in the back seat of a cop car Sunday morning ... minutes after the "Dazed & Confused" actor was punched in the face in a bar fight ... this according to a police report obtained by TMZ.

Here's what the report says ...

Cops were called to the Martini Ranch bar in Scottsdale, Arizona at 1:42 AM on Sunday morning after getting a call about a man who was bleeding on the street.

When officers arrived to the scene ... they found Jason nursing a swollen eye. Cops spoke to a bouncer who told them Jason had been inside the club, walked by him and sneezed on him.  The bouncer asked for an apology, and Jason allegedly punched him in the face.