The 22-year-old was leaving Heart nightclub in the German city when a man aggressively shoved him, but Bieber was quickly removed from the area by a woman in a German Beer Wench outfit.
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"The man was trying, but failing, to get Justin's attention all night and once they came face-to-face he decided to lunge at the singer," said a source from the nightclub.
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In a video of the incident, Bieber's business partner John Shahidi can be seen shoving the attacker to the ground.
IANS