Justin Bieber is NOT going to be in a good mood after this ...

The Biebs was rear-ended by a paparazzo (not ours!) just moments ago in West Hollywood and got a nice scratch on the bumper of his red Ferrari. Neither party was injured and police are still talking to both Justin and the photographer. A pap at the scene tells us he thinks Bieber was trying to be annoying to the photographer following him by not just tapping the breaks, but slamming on the breaks, which kinda worked against him when the following photographer, in a Prius, bumped into Biebs.

A spokesperson for the West Hollywood sheriff's department tells us police are still investigating, fault has not yet been determined.

Shortly after the crash, Bieber took to Twitter to comment, likening the incident to Princess Diana's tragic death.

We get it -- the paps should be more careful. But maybe the 20-year-old shouldn't play games on the road, especially when he's riding around in a $660,000 car!