Jeff Bezos, Lauren Sanchez Wedding Guests Must Go Phoneless for Mystery Island Party

Jeff Bezos and Lauren Sanchez have a big surprise for their A-list wedding guests -- whisking them away to a top secret location, but before they depart they gotta part ways with their phones ... TMZ has learned.

The wedding festivities are in full swing right now ... with some of the celebs sitting down for a fancy rehearsal dinner ahead. After that, our sources tell us all the guests are heading to a mystery island near Venice -- but, no one's been told where the fun's going down.

We're told most of the Bezos-Sanchez partygoers spent most of Thursday lounging by the pool of the swanky Hotel Cipriani, and some others chilled at fabulous restaurants around the Floating City.

But everyone's coming together around 7:30 PM local time. Our sources say all they were told was to be on a particular dock, and ferry boats would take them to an island -- any other details ... only the bride and groom know.

It's a super secretive affair ... we're told guests will have to surrender their phones before they get to the island bash. Yes, that means you too, Kim Kardashian!!!

Lauren and Jeff are expected to welcome around 200 guests to their wedding, and they're reportedly spending a minimum of $50,000 per guest ... do the math, and that's at least $10 million. There's also theme parties -- including pajama and 'Great Gatsby' gatherings.

Celebs we've seen on the ground in Venice include Kim and Khloe Kardashian, Kris Jenner, Orlando Bloom, Scooter Braun, Tom Brady, Oprah and Gayle King.