Former mixed martial arts champion Khabib Nurmagomedov was escorted from a plane after he refused to move from an exit seat.
The Russian, 36, had a disagreement with a flight attendant before take-off on the Frontier Airlines flight which was reportedly flying from Las Vegas to Los Angeles.
In a video of the incident shared widely on social media on Sunday, Khabib is told to either move seats or get off the plane and be rebooked on another flight.
When he questions why, the former undefeated UFC lightweight champion is told: "Because they [the flight attendants] are not comfortable with you sitting in the emergency exit row" and he replies "It is not fair".
He is then warned again and the attendant says she will call a supervisor if he does not move.
Khabib replies that he "would like to sit here" before he is asked to leave by another member of staff. He then calmly leaves the plane.
Khabib is regarded as one of the best MMA fighters of all time and won all 29 of his UFC fights including beating Conor McGregor in 2018.
In a statement on X, Khabib said: "[The lady who asked me] questions was very rude from the very beginning, even though I speak very decent English and can understand everything and agreed to assist, she still insists on removing me from my seat.
"What was the base for that, racial, national or other one, I'm not sure.
"But after 2 minutes of conversation, she called security and I was deplaned from this aircraft, [and later] I boarded another airline and left to my destination.
"I did my best to stay calm and respectful as you can see on the video.
"But those crew members could do better next time and just be nice with clients."
Responding to Khabib's post on X, Frontier Airlines wrote: "We are aware of the incident and are investigating."
BBC Sport has contacted the airline for more details.