TMZ Hollywood Tour

It's the biggest celebrity run-in in the history of all history -- Ben Affleck in his awesome vintage ride ... pulling up to a bunch of unsuspecting tourists and chatting 'em up about Hollywood hot spots.

Oh ... it all went down on a little thing called the TMZ Hollywood tour!! Yeah, we kinda scored.

Talk about bad timing -- the LAPD went full Michael Bay Tuesday on Sunset Blvd ... in one of the craziest busts we've ever seen ... and rudely interrupted the TMZ Hollywood Tour's legendary diarrhea story in the process.

Roughly 10 police cruisers careened to the scene -- law enforcement sources tell TMZ, cops believed they had a crazed DUI suspect on their hands and responded in full force, guns drawn. The footage is intense.

But the sad part -- the bust occurred at the exact moment during the tour ... when we talk about Josh Hartnett's legendary 911 diarrhea emergency at the Chateau Marmont.

The TMZ Hollywood Tour -- the only way to see huge celebrities ... and a well-dressed speed metal fan working up the nerve to ask his gf to marry him. Yeah, that really happened ...

It went down on Rodeo Drive just minutes ago. The bus tour was cruising along ... when they saw Diddy out shopping. So our fearless leader, Harvey Levin, jumped out of the bus and ran to catch up with him.

Turns out Diddy isn't just a fan of Harvey's work on TMZ ... he's a fan of his singing, too! Ok, he didn't actually hear HL's "My Way," .... he heard about it. But still!