Conor McGregor faced a New York judge on Friday morning after being led out of a Brooklyn police station in handcuffs.
The Irishman has been charged with assault over allegations that he and others attacked a bus carrying UFC fighters after a media-day event Thursday in New York City.
The incident left at least two athletes injured, ahead of Saturday night's UFC 223 event at the Barclays Center in Brooklyn.
Police had wanted to question McGregor after footage emerged online appearing to show him throw a "hand truck" at a bus taking rival fighters away from the Barclays Center in Brooklyn.
McGregor, who wasn't on the UFC 223 card, turned himself into the 78th Precinct, in Brooklyn, where he was arrested and subsequently charged with three counts of assault and one count of criminal mischief, the New York Police Department said.