(Reuters) - Amazon.com Inc AMZN.O said on Tuesday its Prime Day sale was on track to be the biggest shopping event in its history by sales.

The amazon echo is seen on display at the Amazon Books store in the Time Warner Center at Columbus Circle in New York City, New York, U.S., May 25, 2017. REUTERS/Shannon Stapleton

The world’s largest online retailer said customers ordered more than three times as many Echo-family speakers than during Prime Day 2016, which at the time broke records for Amazon devices.

Third-party sellers sold over 50 percent more items on the site by noon local time (1900 GMT) than in the same timeframe last year, Amazon said in a statement.

The company has not disclosed sales figures, overall or for the Echo. Analysts estimated last year’s event brought in well over $500 million.

The news underscores Amazon’s break-neck pace of growth as more shoppers order online instead of going to brick-and-mortar stores. It also highlights the winning formula of Amazon’s shopping club Prime, the cornerstone of its business.

Customers had to join Amazon Prime to get discounts during the 30-hour event. U.S. members of the club pay $99 per year for benefits like two-day shipping, and they tend to buy more goods, more often from Amazon. A timer showing when the deals would expire added further shopping encouragement.

Rival retailers like Macy's Inc M.N and Best Buy Co Inc BBY.N took advantage of Prime Day's hype to launch promotions of their own. Still, Amazon often came out on top. Best Buy was 27 percent more expensive for overlapping technology products excluding Amazon devices, according to retail technology company Boomerang Commerce.

Deals on Amazon for the voice-controlled Echo speakers had the promise of boosting its sales indirectly, too. The devices let shoppers re-order products by voice command, removing friction from checkout.

Consumer buzz and heavier discounts likely increased orders for the Echo, Echo Dot and Amazon Tap speakers. Amazon offered the Echo at half price in the United States, or $89.99, versus 28 percent off on Prime Day in 2016.

“The voice-controlled assistant market is hot right now, and by offering a reasonably priced device that does a lot of things - and the promise of many more skills to come - Amazon is positioning itself as a leader in this market,” said eMarketer analyst Victoria Petrock.

Beyond shopping on Amazon, people can tell an Echo to play music, order an Uber ride or turn on the house lights. Research firm eMarketer has forecast the number of U.S. consumers actively using voice-controlled devices will more than double this year to 35.6 million.