Amazon's website was up and down at the start of Prime Day in the United States on Monday at 3 p.m. ET.

The website was experiencing issues like links that appeared to go nowhere, leaving customers unable to access the special deals.

Prime Day is one of Amazon's most important days of the year, when it hosts Black Friday-like deals in the middle of July.

Looks like Amazon wasn't primed for Prime Day this year.

Customers reported experiencing issues with the retailer's website at the same time Prime Day deals were scheduled to begin in the United States, on Monday at 3 p.m. ET, with the site appearing to be down and responding with a 404 error.

Some links on the main page advertising Prime Day deals went nowhere but back to the page. Pages within Amazon's mobile app were also affected, showing an error message with a photo of a dog.

Individual product pages didn't appear to be affected and were still functional, though customers reported issues with adding items to their carts. Some customers also said that their shopping carts would completely empty when they were adding or removing items from it.

Customers reacted to the site issues with anger and confusion, with some even going as far as to threaten to cancel their Prime memberships or shop elsewhere. Many on social media posted pictures of the error pages with the dogs.

"Some customers are having difficulty shopping, and we're working to resolve this issue quickly," Amazon said in a statement at 4:59 p.m. ET.

"Many are shopping successfully — in the first hour of Prime Day in the U.S., customers have ordered more items compared to the first hour last year," the statement continued. "There are hundreds of thousands of deals to come and more than 34 hours to shop Prime Day."

Prime Day is expected to be one of the biggest sales days of the year for Amazon. One Click Retail estimated that Amazon sells about $1 million a minute during peak time on Prime Day, based on last year's data. The issues this year could easily mean millions in lost sales, One Click Retail's founder, Spencer Millerberg told Business Insider at the Digital Food and Beverage Conference in Chicago.

Check out all of our Amazon Prime Day coverage: