Justin Bieber & Sean Kingston -- Call for Back-Up on Miami Beach's Ocean Drive

Justin Bieber and Sean Kingston were just cruising the Miami Beach strip looking like such BALLERS ... they had to flag down a cop to protect them from adoring fans.

Law enforcement sources tell us Biebs was driving a drop-top Rolls-Royce this evening on Ocean Drive -- with Kingston riding shotgun -- when a group of fans noticed them and started to gather around the car.

We're told Justin and Sean got the attention of a plain-clothes officer who stood guard until the crowd left.

Sources tell us the sick Rolls actually belongs to Kingston, who -- for some reason -- is cool with his 17-year-old pal taking the wheel.