The woman was grabbed by the throat by the man in question who wasn't releasing her. Bieber and his friend at the party then allegedly started screaming at the man to let her go.
An unnamed source told the news outlet that the incident took place at party held by Patrick Schwarzenegger, who is a friend of the Canadian pop-star. The Grammy victor and his friend then started yelling at the dude to release her, but he reportedly only screamed back, "Go f*** yourself" as he continued to hold his grip.
"Sources at the party say he appeared to be on drugs - and it looked as if the woman may have been his ex-GF", notes TMZ.
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TMZ sources claim the Baby singer was at the party with a pal when the unnamed guy, who appeared to be on drugs, went insane and attacked a reported ex-girlfriend. Security then escorted the man out of the party.
Authorities eventually arrived and arrested him.
The reports are yet to be confirmed by police or anyone from Bieber's team. He was spotted enjoying Yodeling Boy Mason Ramsey's set at the festival on Friday, and posed for pictures with the preteen afterward.