Justin Bieber vs. Seth Rogen feud: Selena Gomez's ex responds to 'f*ck this kid' diss on Twitter, 'sorry I didn't bow down'

Justin Bieber's biggest hater? Probably "Neighbors" funnyman, Seth Rogen.

The 32-year-old actor's been on a flippant rampage against the 20-year-old pop sensation for months now. During a recent interview with Howard Stern, Rogen exclaims:

"He's a good example of someone who you meet, who you think you're going to hate, and then you get to hate him... He lives up to every one of your expectations."

He also refers to the Biebs as a "motherf*cker piece of sh*t d*ck" - "F*ck this kid!" Zac Efron is SUCH a better example for young Hollywood.

Check it out:

Rogen later went off AGAIN on the "Today Show," insisting:

“To me, the idea that a grown man does not like Justin Bieber is not a controversial thought,” Rogen told host Matt Lauer. “It would be much weirder if I did like Justin Bieber... None of the people in this room like Justin Bieber, I can guarantee you that. I don't know anyone who likes Justin Bieber."

Ouch.

Of course, Selena Gomez's ex couldn't keep quiet for long!

Bieber tweeted on Thursday (May 8):

@justinbieber: "Seth Rogan sorry I didnt bow down when I asked 2 meet u was probably a bit shy and didn't want to be over the top but still. love ur movies."

Rogen has (yet) to respond, but surly he will.

What do Beliebers think?