Law enforcement sources tell TMZ ... the rapper was arrested on Monday afternoon in Downtown L.A. after cops say they found him carrying a concealed firearm, a felony.

We're told Polo G was a passenger in a car that was pulled over, though it's not clear exactly why. In any event ... cops say after searching the car they found the firearm. We're told a male juvenile was also arrested and booked on the same charge.

The arrest comes nearly 3 months after his album release party ... when he was arrested in Miami after allegedly attacking a cop. Cops there say they also found 2 handguns inside a bag after the Cadillac he had been riding in was pulled over for blackout-tinted windows.

Polo G was also a passenger during that stop. Cops said he became combative and they struggled to get handcuffs on him. The escalation left 1 cop injured.