SEATTLE, United States — Amazon.com Inc.'s sales rose sharply during the first three hours of its Prime Day sales event, dispelling fears that the technical glitches that incensed shoppers would significantly hurt business.

Shoppers spent 54 percent more in the first three hours of this year’s event — 3 p.m. to 6 p.m. EST — than in the first three hours of a year ago when the shopping bonanza began at 9 p.m., according to Feedvisor, which sells software to set prices in e-commerce.

Numerous customers vented their frustrations on social media with the hashtag #PrimeDayFail.

Amazon’s annual 36-hour shopathon, an important marketing tool as well as a boost to business, had been expected to drum up $3.4 billion of spending as bargain-hunters chase promotions and discounts. But the e-commerce giant acknowledged Monday that shoppers were having trouble on the site, while numerous customers vented their frustrations on social media with the hashtag #PrimeDayFail. Many complained about being unable to checkout or search for products, instead getting an error page with images of dogs.

Amazon itself hasn’t disclosed specifics, but Feedvisor’s estimates are based on results from its clients. The firm’s data indicates the glitches limited sales in only the first hour of the event, when sales were down 5 percent. They recovered in the second and third hours, according to Feedvisor, which is monitoring the event. Amazon’s stock had slid about 1.5 percent in extended trading in New York.

By Spencer Soper; editors: Jillian Ward, Edwin Chan and Robert Fenner.