Law enforcement sources tell TMZ ... Thomas Dooley was walking down a street Thursday shouting gibberish ... wearing only a small towel wrapped around his waist. We're told some passersby called cops ... and several units rushed to the scene.

Our sources say when officers approached Thomas, he defiantly began walking away, so they tried to detain him. We're told he was taken to the ground and subdued.

That would be it ... it probably wouldn't amount to much more than a misdemeanor, except for the fact an officer injured his knee during the takedown. That elevated things to felony status, so Dooley was booked for felony resisting arrest.

Cops say he was unsurprisingly under the influence of drugs.

Dooley is the son of Meghan's half-brother, Thomas Markle Jr.

He's currently being held on $25k bail.