Footage of a man being forcibly removed from a flight in the US because the airline had overbooked has been shared widely on social media.

Video from onboard a United Airlines flight shows the passenger being dragged from his seat before hitting his head against an arm rest - seemingly leaving him unconscious.

The man is then pulled by his arms along the floor of the plane towards the exit shortly before departure from Chicago to Louisville on Sunday.

A number of videos which captured the encounter appeared on social media, where four passengers had been asked to voluntarily leave the aircraft because the flight had been overbooked.

Fellow flyers said the man forcibly removed from the flight was a doctor who was required to work the next day.

Footage of the incident has been re-tweeted and shared on Facebook thousands of times.

One passenger, Audra D. Bridges, wrote on Facebook: "United Airlines overbooked the flight. They randomly selected people to kick off so their standby crew could have a seat.

"This man is a doctor and has to be at the hospital in the morning. He did not want to get off."

In the video passengers could be heard protesting and shouting at security as they grappled with the man.

One woman, on seeing the passenger knocked out, says: "What are you doing? No, this is wrong. Look at what you did to him."

Ms Bridges added that the incident left passengers "shaky" and "disgusted".

A fellow flyer, Jayse D. Ansprach, wrote on Twitter: "United overbooked and wanted 4 of us to volunteer to give up our seats for personnel that needed to be at work the next day."

He continued: "No one volunteered, so United decided to choose for us. They chose an Asian doctor and his wife.

"The doctor needed to work at the hospital the next day, so he refused to "volunteer." United decided to use force on doctor.

"A couple air port (sic) security men forcefully pulled the doctor out of his chair and to the floor of the aisle.

"In so doing, the doctor's face was slammed against an arm rest, causing serious bleeding from his mouth.

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

Mr Ansprach added that the doctor ran back onto the plane with a bloody face shortly after, shouting "I need to go home".

A number of social media users described themselves as horrified by the video.

Others declared that they would boycott the airline in future.

United has since issued an apology, adding that an urgent "review" was being carried out.

Oscar Munoz, United CEO, said: "This is an upsetting event to all of us here at United.

"I apologise for having to re-accommodate these customers. Our team is moving with a sense of urgency to work with the authorities and conduct our own detailed review of what happened".