At a surprise 25th birthday party thrown by her pals at L.A's La Cita, Lady Gaga hit the stage with the restaurant's Mariachi band.
As 60 close friends cheered on at the low-key Mexican hot spot, the birthday girl belted out several numbers, including hits by Motley Crue, Madonna and her own "Born This Way."
Sporting a blonde beehive and a shimmering, tight black dress, "she was onstage for most of the night performing for all her friends," a source tells UsMagazine.com. "No one knew she was going to go up there and perform with them, I think she just loves to be on stage and doing what she does so well."
Guests noshed on birthday cake (there were several) and when she wasn't on stage, Gaga was mingling with the crowd. "She was walking around saying hi to everyone," adds the source. "It was an amazing night."
Check out Lady Gaga's festive rendition of "Born This Way" above.