Apple's iPhone 6 and iPhone 6 Plus Arrive in Stores Today

PHOTO: The first people in the queue, Sam Sheikh, left, and Jameel Ahmed, right, celebrate for the media after buying the new iPhone6, outside the Apple shop in London, Sept. 19, 2014. PlayKirsty Wigglesworth/AP Photo
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The big day is here.

Sept. 19 has been circled on the calendar of Apple enthusiasts who woke up in the middle of the night last week to pre-order the new iPhone 6 and iPhone 6 Plus. Others have spent days camped outside Apple stores for a chance to be one of the first customers to purchase the new smartphones when the doors open at 8 a.m. local time all across the United States.

Apple said a limited supply of phones will be available in its stores for walk-in customers today -- but they'll likely go fast.

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The devices, which boast larger screens, are so popular since their unveiling last week that they have broken all iPhone pre-order sales records. More than 4 million pre-orders for the phones were placed when they went on sale last Friday, according to Apple.

The iPhones are also in high demand from mobile carriers, many of whom projected delivery dates stretching into October.

One of the things that makes the iPhone so hot, aside from being the latest Apple product, is the larger screen sizes.

The iPhone 6 is 4.7 inches, while the iPhone 6 Plus measures 5.5 inches diagonally, giving Samsung, which has dominated the giant phone market, some competition.

The iPhones are also packed with new features, including "reachability." With a double touch, the display can slide down so users can reach "anything" at the top of the phone, an ideal feature for multitasking.

Apple Pay, the company's digital wallet that is slated to make its debut in October, will also be supported on the new iPhones.

The iPhone 6 starts at $199 for a 16 GB configuration, $299 for 64 GB and $399 for a 128 GB configuration, with a two-year contract.

The iPhone 6 Plus starts at $299 (16 GB), $399 (64 GB) and $499 (128 GB). Both phones come in three colors: gold, silver and space gray.

If you're not getting a new iPhone today, be sure to check out the next best thing.

Earlier this week Apple unveiled iOS 8. While the operating system will come installed on the iPhone 6 and iPhone 6 Plus, users of certain earlier generation Apple products will also be able to download it on their devices.

The operating system makeover is being hailed as Apple's biggest ever because it comes with a slew of new updates to popular apps, including messages, photos and the keyboard -- making your old device feel new again.