At least 15 people were in critical condition after the crash at the Reno Air Races, which a spokesman for the event called a "mass casualty situation" in a written statement.
The plane, a P-51 Mustang dubbed the "Galloping Ghost" that was being flown by pilot Jimmy Leeward, crashed into a box seat area in front of the main grandstand about 4:20 p.m. PDT (2320 GMT), said Mike Draper of the public relations firm R&R Partners, which represents the race.
Video apparently taken from the stands and posted on YouTube showed a plane plunging nose-down into the tarmac as spectators were heard gasping: "Oh, my God."
The head of the Reno Air Racing Association, Michael Houghton, put the number of injured at 54 and said the 74-year-old pilot was killed. He previously put the pilot's age at 80.
Stephanie Kruse, a spokeswoman for the regional emergency medical service authority, said 15 of the injured were in critical condition
Renown Regional Medical Center spokesman Dan Davis said among 21 victims received at his facility, at least two were killed in the crash, a man and a woman. They were not identified and it was not clear if Leeward was one of them.
Mark Hasara of Cedar Rapids, Iowa, an Air Force veteran who witnessed the crash, told Reuters, "As soon as I saw his nose pointed at the ground, I knew he wasn't going to recover."
In a June video posted at the website for the air race, Leeward said the Galloping Ghost raced from 1946 to 1950 in the Cleveland Air Races and afterward in other events.
He said his crew cut 3m off the plane's length and made other modifications to improve its aerodynamic abilities and reach speeds of 800kmh.
The Reno crash was the latest in a spate of fatal air show accidents since August.
Last month, the pilot of an aerobatic airplane died in a fiery crash in front of shocked onlookers at a weekend air show in Kansas City, Missouri. In Michigan last month, a wingwalker at an air show near Detroit plunged about 60m to his death as he tried to climb onto a helicopter in midair.
The Reno Air Races, which began in the mid-1960s, features planes facing races at an airfield north of the city.
