The two headliners kicked off the star-studded performance Sunday at SoFi Stadium, going through a few of their hits, including 'The Next Episode' and 'California Love,' -- but in the middle of them doing their thing on stage ... Mr. Jackson swung into action down below!

Fitty turned out to be the big surprise guest that had been rumored for weeks now ... and he performed 'In Da Club' on a set piece underneath Snoop and Dre -- this while hanging upside down.

It made for a ton of fanfare ... and folks cheered the trio on. Soon after, Mary J. Blige came on and went through a quick medley as well, before passing the baton on to Kendrick ... who eventually handed things off to Eminem, pumping up the energy with each performance.

Snoop and Dre eventually took center stage once again ... and ended things on a high note with 'Still D.R.E.' -- and were flanked by all the artists by the finale of the song.

Seems like this one was a major homage to the legacy of Dre, and turned out to be pretty damn rad as a result ... living up to the hype and then some.