"It looked like he was knocked out, because he went limp and quiet," Mr Anspach wrote, "and they dragged him out of the plane like a rag doll."

Mr Bridge's video shows the passenger screaming as officers yank him from his seat. He is then seen being dragged down the aisle on his back by his hands, body limp, glasses askew and shirt pulled up above his navel.

Another video shows him, still disheveled from the altercation, returning to the cabin, running to the back of the plane and repeating: "I have to go home."

A spokesman for United initially said: "We followed the right procedures. That plane had to depart."

Oscar Munoz, CEO of United Airlines' parent company, apologised first in a written statement and then in a letter to employees on Monday evening.

Mr Munoz said he was "upset to see and hear about what happened" at O'Hare. He added, however, that the man dragged off the plane had ignored requests by crew members to leave and became "disruptive and belligerent," making it necessary to call airport police.

"Our employees followed established procedures for dealing with situations like this," Mr Munoz told employees. "While 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."