Ya gotta see the video ... Mason and his younger brother are in the middle of a huge mob of people, with sparklers going off and Mason's bar mitzvah cake being carted over to him when suddenly he rises up ... hoisted on people's shoulders. FYI ... Jewish tradition is to hoist the bar mitzvah boy up on a chair, but hey ... lifting them outright is ok, too.

While it was Mason's bar mitzvah, he shares a birthday with Reign ... so they both got hoisted in the air ... even though Reign's still a few years away from becoming a man in the Jewish culture.

As you see, proud papa Scott Disick is beaming from the sidelines.

Scott rented out Offsunset lounge, a Hollywood hotspot on the Sunset Strip, for the bash.

There were 200 people at the party, including 100 of Mason and Reign's friends. Like any good bar mitzvah, the kids were on the dance floor all night partying with a disc jockey.

Meanwhile, our sources say the parents had a great time watching their kids dance ... all the Kardashians were there, but notable absences included Corey Gamble, Travis Scott, Travis Barker and Kanye West.

Scott's fingerprints were all over Mason's bar mitzvah ... the lettering on the cake and banners hung around the party mirrored the clothing brand Chrome Hearts ... duds Scott likes to wear around town.