Jennifer Lopez -- Packs in Celebs for Vegas Opener

Opening night of Jennifer Lopez's "All I Have" Las Vegas show was packed with A-list celebs who got up on their feet to worship the booty ... and dance.

Seriously, it could've been a Hollywood award show -- Justin Bieber, Scooter Braun, Ryan Seacrest, Rebel Wilson, T.I., Rosie Huntington-Whiteley, Kelly Osbourne, Hoda Kotb, Derek Hough, Nicole Scherzinger and many, many more.

Her "Shades of Blue" co-star Ray Liotta was also there ... and BFF Leah Remini took in her second performance. And onstage, she had Ja Rule, Ne-Yo and Pitbull performing.

She gave 'em what they came for too -- singing, dancing, and yeah ... lots of booty shaking.