Jennifer Lopez's 50th birthday party was insane -- with huge stars, a huge mansion, a huge cake ... and a 6-figure birthday present courtesy of her fiance, Alex Rodriguez!!!

It all went down Wednesday night at Gloria Estefan's waterfront mansion on Star Island in Miami ... where J Lo's famous friends like DJ Khaled, Fat Joe, Ryan Seacrest and Ashanti rolled to party with Jenny from the Block.

43-year-old A-Rod gifted Jennifer a sick red Porsche 911 GTS convertible, worth around $140,000!!!

He also gave a big speech in front of the crowd, telling Jennifer that he loves her deeply and is so happy to be a part of her 50th birthday. It was sweet.

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And, speaking of sweet, the cake was lit too! It was 10 TIERS and it was taller than most of the guests and took 5 grown-ass dudes to carry it into the party!

We're also told it was covered with with 24-karat edible gold and featured her name and the number “50” in Swarovski crystals. And, it was delicious too -- a Divine Delicacies Custom vanilla rum cake with dulce de leche filling. The bakery had no comment.

We're told the total cake cost -- $10,000!!!

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As for the party itself, everyone was turning up ... even her kids hopped onstage and serenaded J Lo with a birthday rap song. It was adorable!

Also, Fat Joe bumped his hit, "All the Way Up" ... and the place went NUTS!!!

Lopez was also gifted a huge portrait of herself that was also bigger than most of the guests!!

The big grand finale was an explosive fireworks display over the water -- it was like the 4th of July all over again!!!

By the way, Jennifer isn't taking too much time to recover ... she's got a concert in Miami on Thursday, Friday and Saturday night, which will close out her tour.

Props to A-Rod for setting the whole thing up and everyone seemed to have a blast -- and now, we're all thinking the same thing in the office ... how are they gonna one-up this party for the wedding bash?!!