McCartney cameos on live 30 Rock

Sir Paul McCartney has surprised viewers in the US by popping up during a live episode of hit TV comedy 30 Rock.

In a break from its usual pre-recorded format, the half-hour show was broadcast from NBC's Studio 8H - otherwise the home of Saturday Night Live - in front of a live studio audience.

Beatles star Sir Paul joined show regulars Tiny Fey and Alec Baldwin, and surprise guests including Mad Men star Jon Hamm, Amy Poehler and funnyman Jimmy Fallon.

The set-up for the episode had boss Jack Donaghy (Alec) delivering the bad news that, as a cost-cutting measure, TGS, the fictitious show-within-a-show produced by Liz Lemon (Tina), would be filmed, not broadcast live.

"From now on," he said, "you write and shoot the season in two weeks, like Wheel Of Fortune and Fox News."

But by the end of the episode, Jack was won over to the importance of live TV.

"TGS is meant to be live," he concluded. "To make it more profitable, we'll just need to do more sponsored product placements. Or I'll just pay you all less."

Whereupon Sir Paul reappeared on stage, looking dazed. "I slipped and hit my head in the bathroom," he said. "Who am I?"

"You're my boyfriend!" Tina's character told him as she gleefully led him away.

The episode was the second venture of its kind for for 30 Rock, which aired live the first time in October 2010. The initial performance went smoothly, with no noticeable technical glitches and one minor flubbed line.