Customers took to Twitter to complain about problems in downloading iOS 7 on to their mobile devices after the update came online at 6pm BST.
The new operating system overhauls the look of iPhones and iPads and brings in a number of new navigation features – the biggest change in the look of the software since the iPhone was launched in 2007.
Some users claimed the download – which required a Wi-Fi signal – was taking a number of hours to complete.
"iOS8 will be available by time iOS7 finish updating," said one user, DJ Melo. Elle Lake (lizzylake) said: "43 minutes remaining to download iOS 7.0 but now 10 minutes later there is an error, and I have to upload it again with another 2 hour wait".
Many found they were unable to download the new software at all, instead getting error messages on their screen.
Sam Allison, tweeting under VF2010, responded to the problem saying: "Guess I'm not the only one getting an Error message trying to Download IOS 7.0 £Annoying".
Conor Rickards, using the handle cajr, opted for irony, tweeting: "Having a great time using IOS 7.0" whilst posting a picture of the error message.
The new system, which includes fresh camera functions and a streaming radio feature, was unveiled at the company's annual Worldwide Developers Conference in San Francisco in June.
It has been designed to make the iPhone appear bigger, with features crafted to take advantage of the entire screen.
Text is said to appear sharper, while a "control centre" on the phone allows users to adjust settings with just one swipe from the bottom of the screen.
Apple did not respond to calls about the matter.
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