SAN FRANCISCO — Gadget lovers still can’t resist the iPhone.
Even as investors fret that Apple’s smartphone is losing its allure amid a bevy of enticing lower-priced alternatives that offer similar features, the iPhone’s call remains seductive.
Apple Inc. on Monday said it sold 9 million units of its top-of-the-line iPhone 5S and less-expensive iPhone 5C during their first three days on sale. That trounced the performance of last year’s model, the iPhone 5, which sold 5 million units in its opening weekend.
The initial sales figures provided the latest testament to the product’s enduring appeal more than six years after the debut of the first iPhone triggered a mobile computing revolution.
The iPhone 5S and 5C’s quick start also surpassed analyst forecasts, which predicted Apple would sell somewhere from 6 million to 8 million models during the first weekend.
‘‘The demand for the new iPhones has been incredible,’’ Apple chief executive Tim Cook crowed in a statement. As part of the company’s effort to drum up interest in its most profitable product, Cook made an early-morning appearance at an Apple store in Palo Alto, Calif., on Friday when the new iPhones first went on sale.
Monday’s positive news helped generate more interest in Apple’s stock. The shares gained $23.23, or 5 percent, to close at $490.64.
Despite that rally, Apple’s stock price remains slightly below where it stood before the company’s Sept. 10 unveiling of its latest iPhones.
The downturn reflects concerns that the new phones won’t be enough to reverse a slowdown in Apple’s revenue growth that has been brought on by tougher competition from Samsung Electronics and others. Apple’s rivals have been gaining market share by selling less expensive phones running on Google Inc.’s free Android software. Those nagging worries are the main reason that Apple’s market value has sagged by about 30 percent, or $200 billion, since last year’s release of the iPhone 5.
Part of this weekend’s higher iPhone sales volume stemmed from Apple’s decision to make the models available on a broader scale. For instance, this marked the first time a new iPhone has gone on sale in China at the same time the product launched in the United States.
But the opening sales numbers also served as ‘‘a powerful reminder of the strength of (Apple’s) iPhone franchise,’’ ISI Group analyst Brian Marshall wrote in a research note.
In another sign of the intense interest in anything new from Apple, the Cupertino, Calif., company also said about 200 million devices have already embraced its latest mobile operating system. The software, iOS 7, is the most radical makeover of Apple’s mobile operating system since the iPhone came out in 2007. Apple released iOS 7 as a free update Wednesday for iPhone models dating back to 2010, iPads dating back to 2011, and the iPod Touch that came out last year.