This footage is making the rounds online, and for good reason ... it's absolutely nuts. A random dude in South Central late last week attempted to derail an L.A. County fire engine that we're told was en route to a car accident just up the block.

Check out the clip ... you see the man literally come out of nowhere as he runs into the street and leaps onto the giant moving vehicle -- and then, he starts reaching inside.

The fire truck backs up -- apparently in an attempt to shake the dude off -- but he holds on tight ... and it appears he was trying to grab at the firefighters inside. Eventually, two firefighters jump out and push the guy off ... giving chase briefly before he runs off.

That's where the video ends ... but law enforcement sources tell us the story was far from over. We're told after the man walked away, he actually circled back yet again ... and hopped on the engine for a second time. Our sources say the fire truck then backed up even more.

Our sources say the dude held on tight and continued going after the firefighters inside, and eventually cops showed up ... whom we're told had to physically take this man down with force. Once he was finally down, our sources he was cuffed and hauled off to jail.

We're told this man is now facing charges of interfering with the discharge of duty by fireman or emergency rescue, as well as a felony assault charge.

In terms of the actual car crash this fire truck was responding to ... we're told they ended up having to send a different unit to attend to it, all 'cause this guy interrupted the whole thing.