(Bloomberg)—Amazon.com Inc. 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% more in the first three hours of this year’s event —3 p.m. to 6 p.m. Eastern Standard Time—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.

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%. Feedvisor’s estimates are based on results from its clients. Amazon recovered in the second and third hours, according to Feedvisor, which is monitoring the event.

“I’m candidly shocked that they’re not prepared for the traffic,” Forrester analyst Sucharita Kodali said. “Unless this is way beyond their wildest expectations, it’s just odd.”

Amazon, No. 1 in the Internet Retailer 2018 Top 500, said sales in the first 10 hours of Prime Day grew at a faster pace than the same period of the event in 2017. The retail giant said it sold millions of devices that work on the Alexa voice-activated platform, with top-sellers including the Fire TV Stick streaming device with an Alexa Voice remote and the Echo Dot voice-activated speaker.

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—up more than 40 percent from last year—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.

As of Monday afternoon, there were 4,670 social media posts about the Prime Day crash. Eighty percent of online sentiment about Prime Day conveyed anger or sadness, according to Crimson Hexagon, which monitors social-media feedback.

“It wasn’t all a walk in the (dog) park, we had a ruff start,” Amazon said in a statement early Tuesday. It didn’t provide other details on what caused the technical difficulties, noting only that it has hundreds of thousands of new deals today.

The problem is most likely a bug in a software update that should be fixed within hours, said Antony Edwards, chief technology officer at Eggplant, which monitors website performance. Amazon rarely has trouble handling high traffic volumes and its security is unlikely to be breached by a hacker, he said.

“Everything we see points to their content distribution network, which is how they get product images and videos around the world quickly,” Edwards said. “Someone has made a change and it’s not compatible.”

The technical issues weren’t limited to shopping. Thousands of people reported losing connections with their Alexa digital assistants via Echo voice-activated speakers and having trouble streaming Prime Video, according to Downdetector.com, which monitors web trouble. Amazon Web Services, the company’s cloud-computing division, reported global problems with its AWS Management Console, one of its tools. Thousands of big companies rely on AWS to run their websites.

Amazon also faced labor issues. The Ver.di services union said workers at six German sites went on strike Tuesday to protest a lack of health benefits. Unions also called for a three-day strike at Amazon’s main Spanish fulfillment plant, seeking new contractual agreements and a salary increase.