Mr Jobs, who resumed work part-time in June after nearly six months of medical leave, received a standing ovation from a packed auditorium as he made his first public appearance in nearly a year.

The 54-year-old appeared gaunt but not significantly worse than during his last public appearance in October 2008.

Wearing a long-sleeved black shirt, jeans and trainers, Mr Jobs said: "I'm very happy to be here today with you all."

"About five months ago I had a liver transplant. So I now have the liver of a mid-20s person who died in a car crash and was generous enough to donate their organs.

"I hope all of us can be as generous and elect to be organ donors."

Mr Jobs' health is the subject of constant scrutiny and speculation after he underwent an operation for pancreatic cancer five years ago.

At the event last night, Mr Jobs unveiled updates to its online music store iTunes, the availability of 30,000 ringtones for sale for the iPhone, and a iPod equipped with a video camera, radio, microphone and speaker.

The night closed with a live performance by Grammy Award-winning singer Norah Jones, who drew her biggest applause when Jobs kissed her on the cheek at the end of her two songs.