The party went down at an event space in Bev Hills and the guest list is exhausting to type ... Kim Kardashian, Jay-Z, Beyonce, Lady Gaga, Salma Hayek, Snoop Dogg, Irmelin Indenbirken, Soleil Moon Frye, Scott Eastwood, Casey Affleck and Caylee Cowan, Kate Beckinsale, Zoe Saldaña and Marco Perego, Jordan Masterson, Luka Sabbat, Lil Wayne, 2 Chainz, Rita Ora and Taika Waititi, Beck, Chris Rock, Corey Gamble, Emile Hirsch and Lukas Haas.

Kate was a glittery standout with ginormous diamonds caressing her waist.

Besides the glimpse we got inside, the ever-elusive Leo evaded paparazzi in a pretty spectacular way -- given it was his party!

We also don't know if his current GF, Vittoria Ceretti was there, but it's a good bet since we didn't see either of the 2 they came together.

BTW ... Salma had a busy night ... she arrived after attending the Baby2Baby Gala -- a charity for underprivileged kids -- where she received the Giving Tree Award.