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, Jason Derulo 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 buddy Diploin 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.