Britney Spears made an unadvertised appearance at a concert in a San Diego nightclub on Tuesday.
It was her first live performance in more than three years.
There had been many rumours that the scheduled shows of an unknown band called The M+M's were only a cover for Spears.
She performed five of her songs, opening with her 1999 breakthrough Number One single '...Baby One More Time'.
She only addressed the crowd once, according to fans, thanking everyone for coming.
In January, Spears wrote on her website: "I look forward to coming back this year bigger and better than ever."
She is currently working on her fifth album, which she started working on last year with R&B singer Ne-Yo and Swedish producer Max Martin.
