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 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 sales records for Amazon devices. Third-parties sold over 50 per cent more units on its site by noon local time (1900 GMT) than in the same timeframe last year, the company said in a statement.

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 US$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.

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