Grateful Dead Accuser -- Drummer BASHED ME in the Tour Bus

TMZ has obtained police documents in the assault case involving former Grateful Dead drummer Mickey Hart -- in which the alleged victim tells police he got his ass kicked on Mickey's tour bus.

According to the document, the accuser told police in Mercer, Kentucky ... he owns the tour bus and was renting it to Hart.

The accuser told cops Hart's manager had called him over to the bus on Sept. 7 because Mickey wanted to "talk" with him.

When the accuser entered the bus, he claims Mickey directed him to the back section ... shut the door behind him ... and proceeded to beat the living crap out of the guy.

The accuser does not explain why Hart allegedly exploded ... but says the rocker "threw me down, started [to] cuss [at] me, hitting me, pushing me around then told me to get off MY bus."

We broke the story ... a warrant was issued for Hart's arrest.