Apple's new-look operating system has got off to a rocky start, with users complaining of having difficulties downloading it.
Hailed as a new beginning for the technology giant, iOS 7 was eagerly anticipated and millions rushed to download it on its Wednesday release.
But many people complained about long download times, while others had to go through multiple attempts to install the software.
One typical error message read: "Software Update Failed: An error occurred whilst downloading iOS 7.0."
The operating system became available for UK users at 6pm local time but users soon took to social media to express their frustration as "iOS 7.0" became a trending topic on Twitter.
Sam Allison, tweeting under the handle @VF2010--, responded to the outcry, 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 sarcasm, tweeting: "Having a great time using IOS 7.0" whilst posting a picture of the error message."
Others, such as @King--Julien1984, complained about messages telling them their download would take 11 hours, or more, to be completed.
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."
@igc223 said: "Deleted all my apps, music, and pictures and still don't have enough storage to download iOS 7.0"
But some people did manage to download the system and praised it, such as Ashlyn Tracy (@MsPrettyPricey), who said: "Loving the new iOS 7.0 #iPhone. Get it people."
Apple did not immediately respond to a request for comment on the iOS7 download problems.
Meanwhile, many other users who did get to install the update have reported worse battery performance.
"iOS7 is pretty but bye bye battery haha," said Arica (@thisisterrifyin) on Twitter.
"I'm seeing serious battery drain issues with #ios7," added @KevinBuchan.
Using the settings menu to disable features such as Background App Refresh and unnecessary location services is thought to help the problem.
Luke Westaway, from technology site CNET UK, told Sky News that people still to download the update should consider waiting a few days until demand starts to decline.
He said: "You're going to see all of this pressure from other people trying to get at it completely ease off and also that will give other people time to identify any glitches or flaws - which aren't uncommon when an operating system is released."
Industry observers had suggested the arrival of the operating system, which is said to have a cleaner look than its predecessors, could go some way to silencing Apple's critics.
It was unveiled just months after Apple posted its first profit slide in a decade and drew accusations that it had failed to innovate.
British design chief Sir Jonathan Ive called the operating system an "important new direction" when he showcased the software at Apple's annual Worldwide Developers Conference in San Francisco earlier this year.
The American company's chief executive, Tim Cook, described it as "the biggest change to iOS since the introduction of the iPhone".
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.
This gives instant access to functions such as airplane mode, wi-fi, bluetooth or do not disturb, and enables users to quickly pause or play a song, jump to the next track and stream music.
Meanwhile a "notification centre" is available from the "lock" screen so users can view updates with a "simple swipe".
Apple has also introduced an AirDrop tool to share content - said to be fully encrypted - with contacts nearby, and has added further updates to its cameras and its Siri feature.
The new operating system's launch this week comes just days before two new iPhones go on sale - one of which features a fingerprint scanner.
Executive Phil Schiller sent a massive cheer through the audience at the San Francisco conference in June when he told developers: "Can't innovate any more, my a***!"