United Airlines says a man wouldn't give up his spot on an overbooked flight.

So, according to witnesses and videos of the incident, he was pulled screaming from his seat by security, knocked against an arm rest and dragged down the aisle and back to the terminal at Chicago's O'Hare International Airport.

The incident - which United's CEO now describes as an 'unsettling event' - horrified other passengers on the Louisville-bound flight. An airline spokesman apologised for the overbooked flight, and said police were called after a passenger "refused to leave the aircraft voluntarily."

What followed was captured on cellphone video by at least two passengers.

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Tyler Bridges recalled trouble starting almost as soon as he and his wife boarded.

An airline supervisor walked onto the plane and brusquely announced: "We have United employees that need to fly to Louisville tonight. ... This flight's not leaving until four people get off."

"That rubbed some people the wrong way," Bridges said.

Passengers were offered vouchers to rebook, he said, but no one volunteered.

So the airline chose for them.

A young couple was told to leave first, Bridges recalled. "They begrudgingly got up and left."

Then an older man, who refused.

"He says, 'Nope. I'm not getting off the flight. I'm a doctor and have to see patients tomorrow morning,'" Bridges said.

The man became angry as the manager persisted, Bridges said, eventually yelling. "He said, more or less, 'I'm being selected because I'm Chinese.'"

A police officer boarded. Then a second and a third.

Bridges then began recording, as did another passenger - as the officers leaned over the man, a lone holdout in his window seat.

"Can't they rent a car for the pilots?" another passenger asks in the videos.

Then the man, out of frame, screams.

One of the officers quickly reaches across two empty seats, snatches the man and pulls him into the aisle.

"My God!" someone yells - not for the first time.

He goes limp after hitting the floor.

"It looked like it knocked him out," Bridges said. "His nose was bloody."

His glasses nearly knocked off his face, the man clutches his cell phone as one of the officers pulls him by both arms down the aisle and off the plane.

"This is horrible," someone says.

"What are you doing? No! This is wrong."

And with that, Bridges said, four United employees boarded and took the empty seats.

They were not popular among the passengers, he recalled.

"People were saying you should be ashamed to work for this company," Bridges said.

And it wasn't over.

In another video, the man runs back onto the plane, his clothes still mussed from his forcible ejection, frantically repeating: "I have to go home. I have to go home."

"He was kind of dazed and confused," Bridges said. He recalled a group of high school students leaving the plane in disgust at that point, their adult escort explaining to other passengers: "They don't need to see this anymore."

The airline eventually cleared everyone from the plane, Bridges said, and did not let them back on until the man was removed a second time - in a stretcher.

In the end, Bridges and his wife got to Louisville about three hours late.

"It was a pretty tense flight," he said.

Chicago's aviation department says one of its police officers involved in dragging the man from the flight did not follow standard operating procedures and has been placed on leave.

The department said in a brief statement it did not condone the aviation security officers' actions.

The statement did not release the officer's name and it was not immediately clear which of the three men seen in the now-widely seen video taken by another passenger which one was placed on leave.

United CEO Oscar Munoz said the event was "unsettling".

"Our team is moving with a sense of urgency to work with the authorities and conduct our own detailed review of what happened. We are also reaching out to this passenger to talk directly to him and further address and resolve this situation."