Bruno Mars Performs at L.A.'s Intuit Dome, Lady Gaga's Surprise Appearance

Bruno Mars christened the new Intuit Dome in Inglewood, but not without snags -- and there were plenty of celebs on hand to take in the music and the glitches.

First thing's first .. Lady Gaga made a surprise appearance to the delight of the packed house.

The audience was packed with celebs ... Ashton Kutcher, Mila Kunis, Angela Bassett, Courtney B. Vance, Meghan Trainor, Kelsea Ballerini, Chase Stokes, Simu Liu, Halle Bailey, Sophia Bush, Victoria Justice, Derek Hough and more were front and center for night one of Bruno's 2-night gig.

Jennifer Lopez was among the celebs in the crowd ... choosing to spend estranged hubby Ben Affleck's birthday cheering on BM.

Gaga, who Bruno introduced as "pop royalty," rocked a massive blonde wig, white tights, and an itty-bitty pink-and-white dress ... she and BM performed their new duet, "Die With a Smile," onstage for the first time.

Still, we're not sure all the star power will be enough for fans to forget the headache they endured leading up to the show.

Despite the venue's $2 billion price tag, it appears it didn't have working ticket scanners for Bruno's concert ... and thousands of fans were briefly trapped outside amid technical issues.

In addition to problems with facial recognition software, fans claimed they had issues transferring their tickets from Ticketmaster to the Intuit app.

The tech drama delayed the show by 40 minutes ... Bruno didn't hit the stage until 9:40 PM.

The hiccups didn't stop there ... later in the evening, a distressed fan required assistance ... but security was slow to respond, forcing Bruno to call them out.

He snapped ... "You guys are security, right? Get your asses in the middle [of the floor], c'mon! Jump over the gate and help them out."

Security eventually cleared the row and helped the fan, who suffered from a medical emergency.