Here's the deal ... back in April, some California lawmakers were so fed up with skateboarders violating stay-at-home orders, they dumped tons of sand into ramps at parks all over the state.

The move worked for over a month ... but on Monday in Venice, skaters took back the place -- removing the sand with nothing more than shovels, brooms, buckets and bare hands.

The scene was all captured on video by multiple witnesses ... showing dozens of people banding together to fish all the sand out of the popular skate park.

One person who was on the scene said of it all, "All the kids need to have this, because if they don't get it, it's going to be a sad day. You don't want that."

By mid-afternoon on Memorial Day, the park had been completely cleared of the sand ... and boarders performed tricks in the bowls well into the evening.

It's unclear if California officials will let the place remain open ... but video Monday captured several police officers who didn't stop the action as they passed by throughout the day.