Lil Scrappy -- LOCKED UP ... After Refusing Urine Test

Lil Scrappy is a lil' jailbird -- the rapper-slash-reality star turned himself in to an ATL jail this morning after cops say he refused to submit to a court-ordered urine test ... TMZ has learned.

We broke the story ... officials say Scrappy violated his probation back in March after a dispute over a pee test. According to court officials, Scrappy tried to turn in a urine sample that was cold to the touch and may have been tampered with ... and when asked to submit another cup, he refused.

The court eventually issued a warrant for his arrest ... and moments ago, the "Love & Hip Hop: Atlanta" star turned himself in.

On his way into the pokey, Scrap was photographed saying farewell to his fiancee Erica Dixon (seen above, holding hands) and his mother, Momma Dee ... who was already sporting a "Free Scrappy" shirt.

Naturally, the VH1 cameras captured the dramatic moment on tape.