Dao, before being dragged off the parked plane, said repeatedly that he was being discriminated against because he was Chinese, according to Tyler Bridges, a fellow passenger on the flight from Chicago to Louisville, Kentucky.

"He said, 'I'm a doctor; I need to see patients,'" said Bridges, a civil engineer from Louisville who recorded much of the incident on his phone.

Shares of United Continental closed down 1.1 percent at $70.71, after falling as much as 4.4 percent earlier. The company shed as much as about $1 billion in market value before ending the day with a loss of about $250 million. More than 16 million United shares changed hands, the most for any session in a year.

The stock is down about 3 percent for the year.

United is also suffering from broader worries among investors about US airline performance.

In the United States, social media outrage continued, with the incident trending on Twitter for the second consecutive day. Many users promoted hashtags #NewUnitedAirlinesMotto and #BoycottUnitedAirlines.

This is the second time in less than a month that United has been caught in a social media storm. In late March, a United gate agent's decision to refuse to board two teenage girls wearing leggings provoked a viral backlash.

Reuters