Snoop Dogg -- 4/20 Party Shut Down

Snoop Dogg's 4/20 party in Hollywood was shut down before the star of the show even arrived ... TMZ has learned.

"Snoop Lion 420 Festival" -- a giant party being held in a Hollywood Hills mansion to celebrate all things cannabis -- started at 11:00 AM ... but quickly crashed and burned thanks to a bunch of fuddy-duddy neighbors.

Law enforcement sources tell TMZ ... multiple neighbors in the surrounding area complained about the noise levels and parking situation caused by hoards of weed-loving partygoers. The fact that the party didn't have a permit didn't help matters either.

Cops arrived and quickly shut it down, putting a kibosh on the all-night festivities.

Our sources say no one was cited.

We're told Snoop arrived to the party just as it was being shut down. Our sources say Snoop spoke to police and was incredibly cooperative and understood why the party was being shut down.