Large retailers, which have more than $1 billion (£803.21 million) in annual revenue, saw a 64% spike in sales versus an average Monday, compared with last year's 54%, according to the report.
Smaller retailers, with less than $5 million in annual revenue, saw a 30% increase in online sales, the report added.
Prime Day has become a major shopping event in the annual calendar for retailers, rivaling the Thanksgiving holiday weekend's Black Friday.
Customers have to join Amazon Prime to get discounts during the 48-hour event. U.S. members of the club pay $119 per year for benefits including one-day shipping, and they tend to buy more goods, more often from the Seattle-based retailer.
Walmart Inc , Target Corp and eBay Inc are among the many retailers offering their own special promotions to coincide with the annual sale, which Amazon has stretched to two days this year.
Target announced deal days which will run on the same days as Prime Day - July 15 and 16. Walmart offered promotions from July 14-17, while eBay announced a July 15 "crash sale."
"Yes, all of these retailers are getting revenue out of it(Prime Day), but the big winner in this is still Amazon," said Guru Hariharan, chief executive of CommerceIQ, an e-commerce sales and advertising platform for consumer brands.
"The holiday was invented by Amazon, it's branded Prime Day, and shoppers don't remember any other word that describes these midsummer deals," he said.
Amazon said in a statement on Monday customers worldwide had purchased millions of Alexa-enabled smart devices since Prime Day's kickoff, and that other top selling items included the Instant Pot DUO Plus 60, LifeStraw Personal Water Filter, and Crest 3D White Professional Effects Whitening Strips.
Amazon shares edged down 0.25% early Tuesday afternoon. As of Monday's close, the stock has risen 34.6% this year.
(Reporting by Sayanti Chakraborty in Bengaluru and Melissa Fares in New York; Additional reporting by Aishwarya Venugopal in Bengaluru; Editing by Shailesh Kuber, Jonathan Oatis and Richard Chang)