Halloween has officially hit Hollywood -- with massive parties to kick-off the holiday -- including star-studded events held by Paris Hilton and Dan Bilzerian.
Paris' party was held at a massive estate in Beverly Hills ... guests included Heidi Klum and her hubby, Kelly Osbourne, Paris Jackson and Ryan Phillippe. Paris was decked out in a super-hero worthy costume.
Bilzerian held his bash at a mansion in Bel Air with celeb buddies Diplo and Jason Derulo in attendance. The party even featured a neon haunted house for brave guests to walk through.
There are still plenty more parties ahead -- with Cindy Crawford and Rande Gerber's Casamigos event Friday -- so keep an eye out for the best-dressed celebs.