The incident happened around noon during an aerial performance by the Ringling Bros. and Barnum & Bailey Circus at the Dunkin' Donuts Center in Providence.
As many as 20 performers were hurt when the scaffolding collapsed, dropping them between 25 and 40 feet to the ground. The nine who were critically injured are expected to survive, fire officials said.
A spectator uploaded this video of the incident to Youtube. Warning, the footage could disturb some viewers:
Witnesses told the Providence Journal newspaper the performers were in the middle of an aerial act, spinning on ropes tethered to the scaffolding, when the structure suddenly fell down.
"They just dropped," Othello Smith, of Providence, told the newspaper.
"It happened so fast you didn't really believe it happened."
The cause of the collapse was under investigation. Ringling Bros. and Barnum & Bailey Circus did not immediately respond to a request for comment.