X17 EXCLUSIVE - Rihanna Returns To Los Angeles After BFF Katy Perry's Vegas Bachelorette Party

And what a wild party it was!

Singers slash BFFs Katy Perry - who's marrying Brit funnyman Russell Brand later this year - spent Saturday night in Sin City with 25 of their closest pals for Perry's bachelorette bash...

The fun started at the Hard Rock Hotel pool with Katy Tweeting: "Signing up for a liver transplant now." Afterward, the group took a stretch Hummer limo over to the MGM Grand for a special performance of KA by Cirque du Soleil. Here's the two backstage:

And that's not all! The rest of the night included dinner at Ago, partying at XS nightclub and even stopping by a strip club - hey, girls just wanna have fun!

After she returned home to LA, we snapped Ri Ri dining at Giorgio Baldi in Santa Monica.

This all comes in the wake of Russell Brand, Katy's future hubby, allegedly assaulting a photog at LAX. Always with the drama!