The crowd gathered at 6 a.m. to watch the planned implosion of the steam power plant owned by Pacific Gas and Electric. The plant had been decommissioned for decades.
Officials said a 1,000-foot perimeter had been set up around the old plant, and the injured spectators were standing beyond that.
After buildings came down in a fiery crash, a police officer heard a 44-year-old man screaming for help and saw that his leg had been blown off. The four other spectators were treated for minor injuries.
PG&E spokesman Denny Boyles said the company hired subcontractors to set up the perimeter and handle the demolition.
"We are deeply saddened that this happened," Boyles said. "We're looking for answers like everyone else."
Cleveland Wrecking Co. of Covina, the main contractor on the demolition, issued a statement expressing sympathy and vowing a thorough investigation but declined further comment.
"This was a terrible accident, and our hearts go out to the individuals who were injured," the statement said. "We will be conducting a full investigation and will cooperate with the authorities."
The Associated Press contributed to this report.