Justin Bieber's Undercover Cop Trouble

Bieber was signing autographs outside of Macy's when the crowd became unruly ... and a plainclothes police officer rushed to help the singer.

Problem is -- Bieber didn't know the guy was a cop -- so when the cop got close to Justin ... Bieber got defensive thinking he was a crazy fan. A member of Bieber's security team immediately grabbed the cop and tried to pull him away. The cop quickly identified himself -- and cited the security team member for disorderly conduct.