R. Kelly Supporter Trolls Prosecutors Outside Courthouse by Blaring 'Step'

R. Kelly's getting loud support outside his sex crimes trial, and federal prosecutors look like they're getting ready for a dance routine ... thanks to one obnoxious fan with great timing.

This video, shot Friday morning outside the federal courthouse in Brooklyn, is classic. Normally, a team of Assistant U.S. Attorneys arriving to wrap up the 4th week of Kelly's trial would be super boring. But, throw a little "Step in the Name of Love" over it and, well ... check it out.

No, prosecutors didn't bring their own entrance music -- there was a fan by the front door of the courthouse blaring Kelly's tunes off his phone. And yes, the prank is probably in poor taste, considering the charges the disgraced singer's facing.

Still, the timing of the attorneys is uncanny ... they really do seem to be stepping on the beat, almost as if a lawyer flash mob's about to break out.

Eventually, we're told a U.S. Marshall came outside and asked the fan to turn it down, but the prosecutors had already gone inside to do their day jobs.