A huge demand for LEGOs and sky-high prices at retail stores are creating a growing black market in SoCal, where thieves are going to great lengths to jack LEGOs from stores.

Law enforcement sources tell TMZ … theft rings are marching into retailers like Target, Barnes & Noble and Walmart and stealing any and all things LEGO, and the crimes are getting more sophisticated by the day.

This ain’t a smash-and-grab type job … we’re told some of the crews boosting LEGOs are now using counter-surveillance technology, plus getaway drivers and lookouts.

Once the thieves make off with the LEGOs, our sources say they turn around and sell them to fences — folks who deal in stolen goods — and the LEGOs are then sold on websites like eBay, Facebook and OfferUp.

As we reported, there was a big LEGO bust in SoCal earlier this month … with nearly 3,000 boxes of LEGO toy sets being discovered by police, resulting in 2 arrests … and that’s just the tip of the iceberg.

If you haven’t been to the toy section lately, LEGO prices are higher than ever … a key factor in the explosion of the LEGO black market.

Things are getting so crazy, we’re told fences are now sending thieves shopping lists full of popular LEGO sets … and the crimes have become a significant money maker.

Meanwhile, our sources say some stolen LEGOs are being peddled at swap meets … and others are even being sent overseas.

Another factor causing LEGO theft rings to grow … most police departments in SoCal are short on manpower, and our sources say violent crimes are getting priority over stolen toys.

We’re told LEGO is working with law enforcement to combat the black market, but it’s unclear to what degree and capacity.

We’ve reached out to LEGO … so far no word back.