The shocking video footage of the incident is sweeping the internet. In it, a male fan is seen reaching inside the rolled down window of Bieber’s chauffeur-driven car and he either touched or grabbed him on the chest, as the singer made his way towards the Palau Jordi venue.
It's difficult to tell, but it also appears that the fan might've touched Bieber's face with the palm of his hand.
Regardless of how the fan exactly touched him, it was clearly enough to provoke the Canadian singer to lash out and leave the startled man bleeding from his mouth.

The 22-year-old erratic superstar has been involved in numerous public spats, most famously with Orlando Bloom when they bumped into each other on a night out back in 2014.
It was alleged that Bloom attacked Bieber over derogatory sexual comments he made about the Lord of The Rings' star's ex-wife, Miranda Kerr. At the time, the tabloids claimed that Bieber boasted to the movie star that he had slept with the Australian model.
In a report in this morning's Spanish newspaper's El Pais, it's noted: "Bieber is fond of boxing and also practices it regularly. Through Instagram, he has shared numerous images and videos of his workouts.
"Among his friends are boxing champion Floyd Mayweather, who retired in 2015. Mayweather, 39, and Bieber, form one of those strange friendships united by eccentricity and a hobby of posing together on social networks."
While in Barcelona, Bieber had the opportunity to fulfill the fantasy of football fans everywhere by training with Messi at Barcelona FC... thanks to his friendship with Neymar. "They agreed (it) in the United States - and have exchanged messages on social networks," points out this morning's edition of the Spanish broadsheet, El Pais.
Bieber brings his tour to Madrid tonight.