Film shows a man being forcibly removed from an overbooked flight.
The man, who according to social media users is a doctor who was on his way home, can be seen being escorted off the plane by staff.
The airline asked passengers to voluntarily leave the plane but this man refused, saying he had to get home.
After this, he was taken off the plane as passengers filmed the incident.
One passenger tweeted: “He told the police and the United employees he had to be at the hospital in the morning to see patients.”
Another passenger said after the incident: “Kids were crying and people are disturbed.”
United Airlines said in a statement: “Flight 3411 from Chicago to Louisville was overbooked. After our team looked for volunteers, one customer refused to leave the aircraft voluntarily and law enforcement was asked to come to the gate. We apologise for the overbook situation.”