The pre-orders indicate Apple may sell nearly 3 million in the first weekend, almost double the number sold in the opening weekend last year, said Mike Abramsky, an analyst with RBC Capital Markets.
The sales also suggest that customers were unfazed by Apple not updating the iPhone's body design.
"The early momentum of the iPhone 4S reaffirms Apple's leadership in smartphones," said Mark Moskowitz, an analyst at JPMorgan & Chase.
The strength of the orders should silence those who said the lack of a redesigned body on the iPhone 4S would deter buyers, he said.
Apple may sell more than 25 million iPhones in the quarter ending in December, making it the "strongest iPhone launch ever", said Gene Munster, an analyst with Piper Jaffray. The iPhone is Apple's biggest source of revenue, accounting for about half of sales.
The new model comes with voice-command features and a higher-resolution camera, and can run as long as 8 hours on one charge. It also has an A5 chip that will make graphics seven times faster than the old processor and an "intelligent antenna system" for improved call quality.
The iPhone 4S will be Apple's first major hardware release since the October 5 death of co-founder Steve Jobs.
For the first time, the device is available from all three of the largest US carriers.
AT&T said that it received more than 200,000 pre-orders for the iPhone 4S in 12 hours, marking the company's most successful debut yet for the Apple device. Verizon Wireless and Sprint Nextel haven't released sales figures.
The phone works with both CDMA and GSM wireless standards, which are used in different parts of the world.
Adding Sprint as a carrier has helped boost pre-orders, said Jeffrey Fidacaro, an analyst at Susquehanna International Group.
Sprint, the third-largest US mobile-phone service, has an opportunity to add 6 million users a year, he said.
The record sales also may be a tribute to Jobs' legacy, said William Choi, an analyst at Janney Montgomery Scott.
"It's the last phone developed with Steve Jobs at Apple.
"There are definitely some sentiments at play here."
- Bloomberg