US-based Amazon customers are reporting problems completing online purchases on Prime Day, a discounted-shopping event to promote its subscription scheme
Amazon's Twitter account said it was working to resolve difficulties customers were having checking out.
The promotion ran for the first time last year, when the site also faced technical problems.
More than 34.4 million items were sold that day, exceeding the amount sold on Black Friday the previous year.
This year, customers complained of not being able to check out their purchases, with some missing deals available for a limited time.
Others said they were experiencing issues adding items to their cart.
It is not uncommon for sites to struggle under unprecedented volumes of traffic while running promotions.
Many websites suffered problems during last November's Black Friday online rush, including Argos, Boots and Tesco.