Amazon customers are shopping at "record levels" worldwide this Prime Day, Amazon said Tuesday morning.
The 30-hour event began Monday night at 9 p.m. ET. By 4 a.m. Tuesday, Amazon said that millions of Prime members globally had already shopped the website for bargains and "lightning deals."
Amazon didn't offer details on what it called record levels of shopping — whether it represented the number of shoppers or the amount sold.
In the U.S., the Amazon Echo was the best-selling item by 4 a.m., Amazon said. The Echo was selling for $89.99 on Prime Day, half the usual price. Other best-selling deals include those for Amazon's Echo Dot, a 23andMe DNA test and Amazon's Fire 7 Tablet with Alexa.
To take advantage of the discounts, shoppers must be Amazon Prime members, which costs $99 a year and includes perks like free, two-day shipping, access to Prime Video and Prime Music.
Amazon's stock closed Monday up 1.8 percent. Shares were falling 0.8 percent Tuesday afternoon, not moving much one way or the other.
The shopping event, in its third year, is known for offering shoppers some of the best deals on Amazon's own products — particularly electronics. In 2016, Prime Day was the "biggest day ever" for Amazon devices globally and for each Amazon device category, including the Fire TV, Fire tablets, Kindle e-readers and Alexa-enabled devices, Amazon said.
This year, new deals are hitting Amazon's website as often as every five minutes and will be available to Prime members until 3 a.m. ET Wednesday.
As of Tuesday morning, some products on Amazon.com had already sold out, putting customers on waiting lists as they scoured the best deals on everything from toothbrushes to laptops, Under Armour gear to trail mix.
Many of Amazon's competitors, meanwhile, were seen hoping to capitalize on the heavier-than-normal online shopping traffic Tuesday, rolling out their own deals. Some even chose to take a swing at Amazon. EBay, for example, had a banner on its website reading, "Their Prime Deal Is Our Everyday Deal."
"This [day] is really about seeding the market," Piper Jaffray analyst Michael Olson told CNBC's "Squawk on the Street." "Discounting the Echo devices, discounting grocery ... is driving future growth."
By getting so many Echo devices into homes across the world, Amazon is going to be "light-years ahead of the competition," Olson said. "This differentiates [Amazon] from Wal-Mart."
Further, Amazon's Prime Day also offers the internet giant the perfect opportunity to grow its membership base, Cowen & Co. analyst John Blackledge said in a "Squawk on the Street" interview. Amazon reportedly has some 79 million Prime members, on pace to become more popular than cable TV.
"[This day] is about growing Prime adoption. ... We think Prime Day has jump-started Prime penetration," Blackledge added.
Using the hashtag "#AmazonPrimeDay," many Amazon Prime members took to Twitter Monday evening into Tuesday to express either enthusiasm or disappointment over their finds on Amazon.com.
"We continue to view Amazon Prime Day as a promotion geared towards 1) recognizing and rewarding the loyalty of its Prime members; 2) attracting new Prime members; and 3) growing market share across both new and existing product categories," Moody's retail analyst Charlie O'Shea said in a statement.
With Amazon seen ramping up its "content ecosystem," its tech-heavy devices such as the Echo will become more "embedded" in Prime members' lives, O'Shea added.
"Like most retail promotions, profitability really isn't the focus here, but rather topline sales growth, market share gains, and increasing and deepening the Prime membership base are the primary goals..."
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Correction: This story was revised to correct that Amazon Prime Day ends at 3 a.m. ET Wednesday.