An NBC publicist said the alarm went off during the first commercial break and the audience was evacuated for about 20 minutes before being permitted to return to the studio, where the show resumed.

The audience got an unexpected bonus from one of Leno's guests, pop music icon Prince, who played three songs -- two more than the show's musical guests typically play -- said NBC spokeswoman Tracy St. Pierre.

Prince performed "Turn Me Loose," a new, unreleased song, and then after the show wrapped he surprised the audience by performing two private encores -- "Guitar" and the classic "1999."