Videos of the pair were captured by fans who danced the night away and have since circulated on social media platforms like Instagram and Twitter.
Fashion blogger Rebel Neda, who was present during the celebration, told Fox News that Lopez and Affleck arrived at around 9 p.m. with four people, including two bodyguards and a personal photographer.
"They were all drinking and being happy, kissing all the time and super in love. They were very friendly and the Opera shop gave them two bags of presents from the artist Philip Shagnti collection," Neda wrote in an Instagram direct message. "Opera is a very professional and crazy place at the same time and Ben and JLO felt very comfortable and had the time of their life with the crazy personal birthday show that Opera gave them. They got a lot of attention but they loved it."
Neda and other JLo fan pages shared Instagram Stories showing Lopez and Affleck’s visit was filled with excited partygoers.
In many of the clips, Lopez and Affleck remained seated in their private booth while they were greeted with a special dance performance to Lopez’s 1999 hit "Let’s Get Loud."
The over-the-top celebration also included sparklers, champagne, flashing lights, dramatic fog, a Puerto Rican flag and a birthday sign.
"I don’t know exactly how many people there were but a lot! And they all enjoyed it," Neda told Fox News. "They stayed until the closing time 3:00 [a.m.]."
At one point throughout the night, Affleck cozied up to Lopez to whisper something in her ear while her 2002 hit "Jenny from the Block" played in the background.
Neda captured the moment and shared it with her 23,700 followers, and it didn’t take long for the moment to go viral with several fan pages reposting it to their accounts on Twitter, Facebook and Instagram.
"It’s crazy seeing my videos go viral because I totally didn’t expect that at all," Neda shared. "I was just partying like every day at Opera!"
The lavish birthday party came hours after JLo confirmed publicly for the first time she's dating Ben Affleck with a sizzling bikini yacht picture.