He said the aim was to make the system look "cleaner" to help "elevate" users' content. It involves a shift away from skeuomorphism - the use of leather, wood and other real-world inspired textures and artifacts in apps.
A similar change has also been made to the OS X system for Mac computers.
Highlighting the new look of the firm's Game Center app, Apple's senior vice president of Software Engineering Craig Federighi joked: "We just completely ran out of green felt and wood - this has got to be good for the environment."
He also showed off a parallax effect which means icons shift against the background image as an iPhone-user tilts their handset one way and another, based on feedback from the device's accelerometer sensor.
"The new version is almost unrecognizable, which will make it polarising," said Jan Dawson, chief telecoms analyst at the consultancy Ovum. "Some people will love that their phone feels new and different, while others will be disoriented by the newness. Finding your Settings app is hard when the icon has totally changed, and the many people who easily get disoriented by their gadgets may well have a negative experience. On the other hand, this is a clear statement from Apple that it acknowledges the need to refresh the user interface and is willing to do something pretty dramatic."
