At a performance of the musical "Hamilton" on Friday night at the Orpheum Theater in San Francisco broke out mess after the audience had misunderstood a medical emergency for a shoot.
One woman had a heart attack and someone opened an emergency defibrillator, which sounded the alarm when the rifle hit the stage, as part of the duel scene of the show, city spokesman Jonathan Baxter said.
The scene depicts the deadly duel between Alexander Hamilton and Aaron Burr.
"We all ducked and ran out of the theater, one person called" Gun, "tweeted Shirin Rajaee, a reporter for CBS Sacramento, who was in the audience.
Another theatergoer named Marianne Favro tweeted, "Dozens of us hid behind the seats until the police cleared us out – very scary."
Three people were injured when panicked spectators stormed out of the theater. One person broke a leg and two others had moderate injuries, Baxter said.
The woman who had a heart attack was revived and stayed in hospital in critical condition on Saturday, he said.
The performance did not go on, but people were allowed to go back to the theater to take their things with them.
The owner of Orpheum said the audience had been invited to another performance of the popular Tony Award-winning show.