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A video posted to Twitter on Sunday shows a passenger on a United Airlines flight from Chicago to Louisville being forcibly removed from the plane before takeoff.

According to witnesses, United had overbooked the flight, and they needed four seats for their stand-by crew.

When no one volunteered to give up their spot, a United manager came onboard the aircraft and said four "volunteers" would be randomly selected by computer.

After one couple was selected and peacefully gave up their seats, a man was picked next, but he refused to leave.

According to witnesses, he said he was a doctor and couldn’t take a later flight because he had patients to see at his hospital in the morning.

At that point, security officers were called onboard the plane.

The Twitter video shows three men wearing radio equipment and security jackets speaking with the man as he refuses to get out of his seat.

After a few seconds, one of the men grabs the passenger, who screams. The officer drags the passenger by his arms toward the front of the plane, bloodying his face.

Other passengers react with shock and horror as he is dragged through the aisle.

The flight eventually took off, about two hours late.

The video quickly went viral, and people took to social media to express their disgust with United:

A United spokesperson confirmed in an email Sunday night that a passenger had been taken off a flight in Chicago.

"Flight 3411 from Chicago to Louisville was overbooked," the spokesperson said. "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 apologize for the overbook situation. Further details on the removed customer should be directed to authorities."

A statement from United CEO Oscar Munoz was later posted to the United Facebook page:

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