Media playback is unsupported on your device Media caption An eyewitness describes how the passenger sitting next to her was dragged off a United Airlines plane.

Shares in United Airlines' parent company have dropped after footage of a screaming passenger being dragged off an overbooked plane went viral.

Stock in United Continental Holdings dropped by more than 4% at one point on Tuesday, and at one point nearly $1bn (£800m) was wiped off its value.

Its CEO defended employees' conduct and said the passenger had been "disruptive and belligerent".

But the company has come under intense criticism online.

Outraged customers have threatened in droves to stop flying with United and the CEO's response to the incident has been pilloried.

Share prices recovered slightly later in the day, but were still below opening prices.

Media playback is unsupported on your device Media caption John Klaassen, eyewitness: "They treated him as less than human"

The footage taken inside the airliner shows a man being pulled out of his seat and dragged, screaming, down the aisle. He is later seen with blood on his face.

The man has been identified as David Dao, a medical doctor from Elizabethtown, Kentucky.

A passenger who sat next to him told BBC Radio 5 Live that Mr Dao said he was originally from Vietnam. He said he and his wife were both doctors.

The flight from Chicago to Louisville on Sunday evening had been overbooked - a fairly common practice - so the airline wanted to get four passengers to leave the flight to make room for four staff members.

Image copyright Jayse D Anspach Image caption The man has not been identified but told other passengers he was a doctor, before aviation security officers forcibly removed him

Three passengers - reportedly including Mr Dao's wife - agreed and left the plane. But the man said he worked in a hospital, and needed to see his patients the next day, an eyewitness tweeted.

A video that appears to show the man dazed and with blood around his mouth, saying "just kill me", having run back on the plane, has also emerged online.

United Airlines incident: What went wrong?

In an email to employees, Oscar Munoz said the passenger had been "disruptive and belligerent" and refused to voluntarily leave the plane.

Employees were "left with no choice but to call Chicago Aviation Security Officers to assist in removing the customer from the flight", he added.

He told staff in the private email that he was "upset to see and hear about what happened" but defended United.

"Our employees followed established procedures for dealing with situations like this," the Associated Press quoted the email as saying.

United Airlines' public relations disaster

Only last month, trade magazine PRWeek named Mr Munoz as 2017 US Communicator of the Year.

But the airline's handling of the fallout from this incident has been roundly mocked on Twitter.

Users are posting under the hashtag #NewUnitedAirlinesMottos and the Merriam-Webster dictionary tweeted that volunteering means doing something "without being forced".

United Airlines mocked online

Asian outrage at passenger removal

More than 60,000 people so far have signed a petition to the White House asking for an investigation into the incident.

Image copyright Twitter/@fanqin0619

Image copyright Twitter/@Parveen_Comms

One of the three security officers involved has been "placed on leave", the Chicago Department of Aviation said, and his actions were "obviously not condoned by the Department".

The department also said it would carry out a review into the incident, which it said was "not in accordance with our standard operating procedure".

The US Department of Transportation is reviewing whether United complied with rules on overbooking.

Canada to tackle overbooking on flights

"While it is legal for airlines to involuntarily bump passengers from an oversold flight when there are not enough volunteers, it is the airline's responsibility to determine its own fair boarding priorities," a spokesperson said in a statement reported by Reuters.

United said it was trying to talk to the passenger directly in order to "further address and resolve this situation".

Would you change your flight for the right offer?

Can an airline really treat passengers like this? - by Simon Calder, travel correspondent for the Independent

Yes. The captain is in charge of the aircraft. And if he or she decides that someone needs to be offloaded, that command has to be obeyed. From the moment that the unfortunate individual in this case said, "I'm staying put", he became a disruptive passenger.

From that moment he was disobeying the captain's command. Officials were legally entitled to remove him, and as the videos show, he was dragged from the plane. It appears from the evidence that the law was broken - by him, not by the airline. But I would be surprised if United pressed charges.

Does it happen often?

No - normally airlines handle cases of too many passengers for the available seats much better than this, and generally do so at the gate. First, the airline asks for volunteers. The idea is that everyone has their price: an amount of cash, travel vouchers or other bribes such as a round trip anywhere the airline goes.

Flexible travellers, including me, actively pursue overbooked flights to keep our travel costs down.

So what went wrong here?

It appears to have been a series of errors. A group of flight crew needed to be in Louisville, properly rested, in order to operate the next morning's plane. Had they not been able to get there, then many more passengers would have had their plans messed up. The big mistake the airline made was allowing all the fare-paying passengers on board, and then trying to entice enough people off.

It would have been far better to conduct the auction at the gate; physically preventing someone boarding is less harmful than dragging them kicking and screaming from their seat.