And it appears that many of them are coming up empty-handed and frustrated.

Many shoppers took to Twitter on Tuesday to say they were struggling to make it through Amazon’s digital checkout process. In particular, they complained that they are getting an “add to cart fail” message each time they try to scoop up one of the deals. Amazon acknowledged the complaints on Twitter, saying “Some customers are reporting difficulty with checkout. We’re working to resolve the issue quickly.”

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Social media was brimming with messages from steamed customers who couldn’t get the goods they wanted.

And yet, despite those complaints, there are early signals that Amazon is ringing up plenty of sales. Amazon said that in the first eight hours of the sale, U.S. customers had purchased more than 196,000 pairs of shoes and more than 270,000 toys. Many of its limited-time “lightning deals” were selling out quickly.

This is the second year that Amazon has held Prime Day, after first trying the event last summer to commemorate its 20th anniversary. This year, by bringing it back, it is effectively testing of whether the bonanza of deals can become a summertime ritual for shoppers — and a reliable, annual magnet for attracting new Prime members and ringing up big sales. (Jeffrey P. Bezos, the chief executive of Amazon, owns The Washington Post.)

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Amazon has been working in overdrive to generate interest in Prime Day: Since July 5, it has been offering a smaller slate of daily “countdown” deals on items such as vacuums, protein supplements and laptops. As part of a tie-in with its Prime Music streaming service, it is running a contest in which customers can enter to win meet-and-greets with artists such as country singer Carrie Underwood and rapper Flo Rida.

It’s not hard to see why Amazon is pushing hard to drum up interest in Prime Day. During last year’s event, the company reported that customers purchased 34.4 million items — more than any other Black Friday in the company’s history to that point. (It’s noteworthy that the company did not offer comparisons to its Cyber Monday results, suggesting Prime Day sales did not eclipse those on the holiday season’s biggest day for online shopping.)

And, crucially, it reported more people signed up for Prime that day than on any other day in Amazon’s history. This, perhaps, is even more valuable than having a record sales day, because roping customers into the membership program could be an effective way of generating repeat business.

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According to researchers at the e-commerce analytics firm Slice Intelligence, 3 percent of shoppers who made purchases on Prime Day in 2015 had not shopped on Amazon in the six months leading up to the eventbefore that. But in the six months afterward, that group went on to spend $40 millon on the site.

Prime Day has prompted a wide spectrum of reaction from rival retailers. Just as it did last year, Walmart has made an especially aggressive play to go toe-to-toe with Amazon, announcing that it would offer free shipping from July 11 through July 15 on all purchases, a change from its usual policy of offering free shipping on orders over $50. Walmart is also touting special deals on items ranging from a Samsung 4K TV for $598 to a women’s denim jacket for $8. And Walmart is trumpeting 30-day free trials of its pilot Shipping Pass program, a Prime-like offering in which customers can pay $49 per year for free two-day shipping on most online orders.

On Monday, J.C. Penney took to Twitter to tout a big sale, making a not-so-subtle reference to Prime Day by saying, “Our prime deals start today, so you can snooze through tomorrow.” By late Monday evening, Best Buy’s homepage promised, “Deals for all. No membership needed” — a clear dig at Amazon’s sale.

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Toys R Us is having a sale Tuesday, offering 15 percent off regular priced items and sale prices on more than 50,000 items. An e-commerce player, NewEgg, is also aiming to get in on the action by offering hundreds of deals on Tuesday on items such as laptops and smartphones. However, major retailers such as Target are sticking with their usual retailing playbooks this week.

In some ways, Prime Day has plenty in common with the kinds of annual sales that traditional retailers have held for years. Mattress chains, for example, have long packed three-day weekends such as Memorial Day with promotions. Department stores such as Bloomingdale’s have annual “Friends and Family” sales that are highly-anticipated by loyal customers. But Prime Day is unique in its scale: For example, Amazon is pledging to have an inventory of discounted TVs that is almost double what it would have on Black Friday and Cyber Monday combined.

One good sign for Amazon: Last year, many Prime Day shoppers complained that the mix of merchandise had all the appeal of a neighborhood garage sale. This year, there are some hints that customers were more satisfied. Adobe Digital Insights tracked social media sentiment around the sale compared with last year and found that messages with a tone of “sadness” were down 26 percent vs. the day of the sale last year. Meanwhile, messages related to “joy” were up 43 percent. The messages that were classified as expressing “sadness” were largely related to the inability to make it through checkout.