Passengers on a United Airlines flight say a man claiming to be a doctor was dragged off the plane before takeoff, after no one volunteered to get off the overbooked flight.

The incident happened Sunday, at Chicago's O'Hare International Airport, on a flight bound for Louisville, Ky. United says airline representatives asked for volunteers to take a later flight, and when no one came forward, the airline randomly selected four passengers.

However, one passenger claiming to be a doctor refused to leave, witnesses said.

"He told the police and the United employees he had to be at the hospital in the morning to see patients," passenger Tyler Bridges said in a tweet.

Bridges said United offered US$400, and then $800 vouchers and a hotel stay, to encourage passengers to voluntarily give up their seats.

Multiple videos posted online Sunday show police attempting to wrestle the man out of his seat while he tries to hang on. The man can be heard screaming during the altercation, but the screams stop when one officer finally manages to haul him out of the seat and into the aisle. The man appears to be unconscious after he is pulled from the seat, although it is not clear how it happened.

Both videos show him being dragged down the aisle by his arms, with his eyes closed, face expressionless, belly exposed and glasses hanging from his face. One video shows what looks like a drop of blood at the edge of his mouth.

"Oh my God, no!" a woman can be heard saying, while the man is dragged out. "This is wrong!"

United Airlines CEO Oscar Munoz apologized in a written statement and a separate letter to employees on Monday.

"I apologize for having to re-accommodate these customers," United Airlines CEO Oscar Munoz said in a . "This is an upsetting event to all of us here at United."

Flight 3411 was operated for United by Republic Airline, which United hires to fly United Express flights.

Munoz said four Republic employees approached United's gate agents after the plane was fully loaded, and said they needed to board. After the passenger was removed, the four airline employees took the now vacant seats as passengers jeered.

The removed passenger then somehow returned to the plane鈥檚 cabin, looking dazed by the altercation, and saying he had to get home. Meanwhile, another man travelling with a group of high school students stood up and said they were getting off the plane, Bridges said.

He recalls several other people followed suit before United made an announcement instructing the rest of the passengers to get off so the crew could 鈥渢idy up.鈥�

Bridges said his wife told him she spotted the man who was removed on a stretcher inside the airport. The flight left Chicago after a three-hour delay, he said.

Munoz said he has reached out to the man who was dragged off the United flight, "to talk directly to him and further address and resolve this situation."

He also affirmed his support for his staff, saying 鈥淲hile I deeply regret this situation arose, I also emphatically stand behind all of you, and I want to commend you for continuing to go above and beyond to ensure we fly right.鈥�

United also apologized for the overbooking on Twitter earlier in the day.

Airlines routinely overbook flights on the assumption that some travellers will not show up.

In the U.S., when an airline demands that a passenger give up a seat, the company is required to pay compensation.

While no such rules are currently in place in Canada, a spokesperson for Transport Minister Marc Garneau said bumping rules will be included in an air passenger bill of rights that was promised last fall to establish clear, minimum requirements for compensation when flights are oversold or luggage lost.

The incident Sunday comes just two weeks after United barred a teenager from boarding a flight, because her leggings did not meet the dress code stipulated on her employee travel pass.

United faced more public backlash for its latest misstep on Monday, with Twitter users condemning the airline for its seemingly draconian tactics.

Several celebrities also came out against the airline.