Justin Bieber: Cry Baby, Baby, Baby ... Oooh

An emotional Justin Bieber appeared to be fighting back tears mid-song at his Philly tour stop.

The waterworks came on Saturday night while JB was performing "Purpose" ... which he says he wrote for the big man above.

So, educated guess ... his weeping is either based in religion or regret. Seriously, the guy just got a face tattoo -- y'know there's gotta be some regret.