Leo DiCaprio, Usher, Justin Timberlake Hold Court at WME Pre-Oscars Bash

Hollywood's elite are warming up for Sunday's Oscars with pre-parties every bit as celeb-packed as the main event will be ... as Usher, John Legend and Leo DiCaprio rallied in one of the town's more iconic mansions!

TMZ got this video inside the William Morris Endeavor talent agency's big bash in Bev Hills Friday night -- it was held at the Beverly Estate, sometimes referred to as the "Godfather" mansion, and it was absolutely swimming with A-Listers.

Check out these faces in the crowd -- Justin Timberlake, Tobey Maguire, Teyana Taylor, Busta Rhymes, Rob Lowe and, of course, the 3 aforementioned superstars ... all rubbing elbows, literally and figuratively.

Even the celebs themselves seemed starstruck at times ... you can see Justin, Usher and Teyana were posing together for a few snaps as they chatted and laughed.

After their little photo sesh, Justin chatted a bit with some of the other guests as Teyana walked around, mingling.

At one point, Teyana bumped into Busta and the two began to shoot the breeze. Rob was also spotted making the rounds.

Never one to miss a party filled with beautiful people ... Leo, along with his pal Tobey, popped in just as the music started pumping and everyone was hitting the dance floor.

By all accounts, Leo was in good spirits and not even hiding his face that much, as we usually see him.

As for John Legend ... Mr. EGOT got off his feet to do the small talk thing ... sitting on a couch to chat up some other guests.

By the way, the mansion where all this was going down is the spot where they filmed the famous horse's head scene for "The Godfather" flick.