The Seattle, Washington-based online retail giant had initially planned to ship the Kindle Fire, which at $199 costs less than half the cheapest iPad, on Tuesday but it began sending them to customers a day early.
Dave Limp, vice president of Amazon Kindle, said the Fire is already the "bestselling item across all of Amazon.com" and the company was "building millions more than we'd planned" because of the customer response.
As usual, though, Amazon did not provide any actual sales figures.
The Kindle Fire has a seven-inch (17.78-centimeter) screen, smaller than the iPad's 9.7 inches (24.6 cm), connects to the Web using Wi-Fi and is powered by Google's Android software.
It does not have a camera or the 3G connectivity featured on other tablets but it gives buyers easy access to Amazon's online store, which sells books, music, movies, television shows, games and other content.
Technology reviewers were divided in their appraisal of the device after getting their hands on preview models.
David Pogue, the influential technology columnist for The New York Times, said the $200 price tag would attract some buyers but "the Fire does not have anything like the polish or speed of an iPad.
"You feel that $200 price tag with every swipe of your finger," Pogue said, adding that the Fire is "not nearly as versatile as a real tablet."
"The Fire deserves to be a disruptive, gigantic force," Pogue said. "But at the moment, it needs a lot more polish; if you're used to an iPad or 'real' Android tablet, its software gremlins will drive you nuts."
Writing for Wired magazine, Jon Phillips said "the Fire isn't a dud, but its real-world performance and utility match neither the benchmarks of public expectation, nor the standards set by the world's best tablets.
"Fire is a fiendishly effective shopping portal in the guise of a seven-inch slate," Phillips said. "It does nothing very well, save video playback, running various Android apps, and making the business of Amazon shopping alarmingly fun and easy."
Sam Biddle of technology blog Gizmodo said the iPad "finally has serious competition" and that Apple should "be afraid."
"Reading, watching, browsing, and listening on the Fire are all tremendous, easy fun," he said. "It's not as powerful or capable as an iPad, but it's also a sliver of the price."
Apple's cheapest iPad sells for $499.
Reviewer Joshua Topolsky of TheVerge.com said the Kindle Fire "isn't an iPad-killer" but is a "really terrific tablet for its price.
"The amount of content you have access to -- and the ease of getting to that content -- is notable to say the least," Topolsky said. "The device is decently designed, and the software -- while lacking some polish -- is still excellent compared to pretty much anything in this range.
"It's a well thought out tablet that can only get better as the company refines the software," he said. "It's not perfect, but it's a great start, and at $200, that may be all Amazon needs this holiday shopping season."
According to technology research firm Gartner, the iPad will account for 73 percent of worldwide tablet sales of 63.6 million units this year and will remain the top-selling device over the next few years.
Amazon shares were up 1.51 percent at $220.68 on Wall Street at mid-day on Monday.