JANUARY 23--Singer Justin Bieber--belligerent and apparently stoned on a mixture of beer, pot, and prescription drugs--cursed out a cop who had stopped his Lamborghini, demanding, “Why the fuck are you doing this?” and “What the fuck did I do. Why did you stop me?” according to a police arrest report.

The 19-year-old performer reeked of booze, had bloodshot eyes, and appeared in a stupor, a Miami Beach Police Department officer reported.

When the cop sought to conduct a pat down of Bieber, the entertainer stated, “I ain’t got no fucking weapons, why do you have to search me? What the fuck is this about?”

After Bieber repeatedly ignored a patrolman’s direction to keep his hands on the vehicle, the cop grabbed the singer’s right arm and told him he was under arrest. This prompted the 5’ 9”, 140-pound Bieber to declare, “What the fuck are you doing?” as he sought to pull away from the officer.

After being stuffed into a patrol car, Bieber was transported to the Miami Beach Police Department. En route, he asked a cop why he had been arrested, and was told that investigators believed he was impaired. Bieber countered that he was not drunk “and that he he was coming back from recording music at a studio.”

At the police station, Bieber failed a series of field sobriety tests. He also agreed to take a Breathalyzer test and submit to a “drug evaluation” (the results of which are not detailed in the police report).

Bieber is seen in the Lamborghini in an Instagram photo (above) posted yesterday by the rapper Khalil Sharief, who was reportedly drag racing with Bieber early this morning. The 19-year-old Sharief, driving a red Ferrari, was also arrested on a DUI count. (3 pages)