Rob Lowe Celebrates 60th Birthday with Star-Studded Party

Rob Lowe's ringin' in a new decade right ... partying hard at a rager in Los Angeles -- with as much star power as the Oscars red carpet.

The film and television star turned 60 last week, but celebrated big by the beach in beautiful Santa Monica this weekend ... and, the guy's got a ton of celebs from his more than 40 years in Hollywood who showed up to get down.

Cameron Diaz and her man Benji Madden -- who just welcomed another kid into the world, BTW -- pulled up to support RL on his big day ... enjoying a nice couple's night out while leaving the kiddos at home.

And, Lowe's "Parks and Recreation" pal Chris Pratt showed up to support his longtime costar ... with wife and -- super celeb in her own right -- Katherine Schwarzenegger joining him.

Even Ellen DeGeneres -- who's been livin' the quiet life with her chickens since leaving her talk show behind -- made it down to the city for the event.

Among the other notable names ... Robert Downey Jr., Gwyneth Paltrow, Katy Perry, Orlando Bloom, Adam Levine, Kenny G, Maria Shriver, Christopher Schwarzenegger, Zoe Saldana, Marco Perego, Laura Arrillaga-Andreessen, Maude Apatow, John Owen Lowe, and Netflix boss Ted Sarandos.

We hear Kenny G actually performed at the function ... and the rager went into the early hours of Sunday morning. Sounds like one hell of a party!

FWIW ... Rob's birthday is actually March 17 -- but, it seems like he spent it hangin' around with the fam in some tropical paradise. This looks like the bigger bash to really honor the man in Hollywood.

Oh, and BTW ... paps caught Rob himself walking around outside -- don't worry if you didn't recognize him -- the guy doesn't look nearly old enough to be 60. A perfect example of aging like a fine wine.