JUSTIN BIEBER flipped out on a paparazzi scumbag on Friday, while leaving his London hotel. Justin apparently shoved the guy, and he started cussing Justin out. So Justin lunged at him, saying, quote, "I'll {effing} beat the {eff} out of you!" But Justin's security held him back, and he eventually got into his car and took off. Meanwhile, Justin has canceled a gig tomorrow night in Portugal due to, quote, "unforseen circumstances."