(Reuters) - An Amazon.com Inc shopping event that the online retailer billed as its largest ever had a bumpy start on Tuesday, with some customers unable to add discounted items to their shopping carts and rival retailers aiming for a cut of the buying frenzy.

The glitch, which affected Amazon's U.S. and UK websites, sparked outrage on social media with the hashtags #PrimeDay and #PrimeDayFail trending in the United States.

Amazon said it resolved the issue, in which users faced problems adding some "Lightning Deals" products to their carts.

"#PrimeDay is giving me nothing but add-to-cart fails. Nice job @amazon. Another #PrimeDayFail," Twitter user Prero Design (@PreroDesign) said in a post.

The snag was a blow to a company that takes pride in its stable technology infrastructure.

BGC Partners analyst Colin Gillis said the glitch potentially indicated a flood of demand for products on Amazon.

The one-day sale for members of Amazon's $99-per-year Prime subscription service had been expected to generate up to $1 billion in sales, more than double the $400 million in extra sales made during last year's inaugural Prime Day, according to Citi analysts.

But by mid-day Tuesday, U.S. sales remained in line with Amazon's 2015 Prime Day, according to a report from e-commerce software company ChannelAdvisor. Sales for the United Kingdom were up 11 percent from last year as of 12 p.m. EDT.

Scot Wingo, executive chairman of ChannelAdvisor, said the UK results "could be indicative of where we end up," with more deals for U.S. customers scheduled to launch this afternoon.

Still, the results are expected to pale in comparison with the more than $14 billion of total value of goods transacted during Alibaba Group Holding Ltd's Singles' Day shopping festival in China in November.

Amazon.com Inc shares were down 0.7 percent at the close of trading, while broad stock market indices were up 0.7 percent.

Amazon devices such as the Echo voice-enabled speaker, Fire TV and Fire tablet were available at big discounts as were a host of other products from high-end televisions to shoes and toys.

Several other U.S. retailers appeared to attempt cashing in on the hype surrounding Amazon's Prime Day event, by offering their own online midsummer promotions.

Wal-Mart Stores Inc launched free shipping with no minimum order threshold on all items, touting products "at or lower than Amazon's prices." Gap Inc, Macy's Inc, Target Corp and others marketed midsummer sales as well.

(Reporting by Siddharth Cavale, Subrat Patnaik and Supantha Mukherjee in Bengaluru; Angela Moon in New York; and Jeffrey Dastin in San Francisco; Editing by Kirti Pandey and Sandra Maler)