For Mac Saturn, Friday night’s show at the Fillmore Detroit had been intended as a celebratory hometown occasion. And when all was said and done, despite a sudden unwelcome backdrop, that’s exactly what it was.

Hours before the biggest and most meaningful concert of the young rock band’s career, members learned along with the public that keyboardist Evan Mercer had been arrested by the FBI on child pornography charges. Mercer now sits in a Detroit federal jail ahead of a Monday detention hearing.

Mac Saturn acted quickly, announcing on social media that Mercer was booted from the band. The post noted he’d been a recent arrival to the group, which is helmed by the founding duo of singer Carson Macc and guitarist Nick Barone and rounded out by guitarist Mike Moody, drummer Angelo Coppola and bassist Ian Lukas.

And so Mac Saturn — now abruptly a five-piece — was off to the Fillmore for its big night marking the release of the debut album “Hard to Sell.” Buzz on the band has been positive and growing in Michigan and beyond, and Friday’s tour-launching concert was to go on undeterred.

While mutual Detroit love was the theme of the night, the show drew concertgoers from places such as Texas, Tennessee, Alabama and Pennsylvania — diehard fans the band has collected along its recent road travels. Some had queued up outside the Fillmore as early as noon, and the disturbing Mercer news that arrived midafternoon hit with a wallop.

But Friday evening inside the theater, where music was to take center stage and anticipation was humming, Mercer’s arrest was the elephant that wasn’t in the room. If anything, fans seemed duty-bound to reinforce their support for a band whose name had been unwittingly attached to a mess. This crowd wasn’t going to let this night be a downer.

One twentysomething identifying herself only as Jenna summed it up: “Right now, that’s all out there, and in here,” she said, patting her cell phone.

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Backstage, according to those on the scene, the mood was upbeat as band members’ family and friends swung by to deliver well-wishes.

Before the group hit the stage, vocalist Macc walked out to address the topic.

“The news of today has been shocking and horrifying,” he said. “We came here tonight to see you guys face to face and play this music we have worked so hard on for you.”

And that’s what they did: After the Four Tops’ “Reach Out I’ll Be There” boomed over the PA for a poignant hometown Motown introduction, Mac Saturn lit into a rollicking hour-plus set of glam-spiked rock with a soul pulse.

Following the frisky “Diamonds” and stylized swagger of “Persian Rugs,” nine songs from the new album lined the set, with touches of back-alley blues (“Get on the Phone”), insta-hooks (“Mr. Cadillac”), atmospheric turns (“Box Cutter”) and endearing fan faves (“Mint Julep,” “Ain’t Like You”).

Album closer “Plain Clothes Gentleman” was also Friday’s show finale, building from swampy slide-guitar wistfulness into a wiry stomp and scorching outro.

It was all delivered with energetic, tried-and-true Detroit showmanship, with Macc’s slinky, Jagger-inspired presence up front bolstered by assorted scarves, furs and leopard-pattern jackets. The rest of the band offered its own punch, with Moody serving up nimble, R&B-infused leads and backing vocals alongside Barone and Coppola.

The crowd gave as well as it got, singing along in full voice for much of the show, including the brand-new stuff.

It capped a day that was already destined to be etched in Mac Saturn’s minds. For bands on the rise, this is the career phase that happens only once, no matter how things build from here: It’s that ripe chapter when momentum is surging but the stages are still small, the crowds organic, the production sparse — like the basic “MAC SATURN” backdrop that accompanied the group Friday night.

The Mercer situation may be a tough reality Mac Saturn is saddled with for a while, a public-relations headache for a likable band that was clearly finding its groove and feeling the wind at its back.

But in the end, it may be no more than a road bump. And the group’s feel-good vibes certainly don’t hurt the cause as it embarks on a 35-city headlining tour in support of “Hard to Sell.”

“We’re going to leave tomorrow morning and play around the country,” Macc told the crowd during the show’s closing stretch. “And we’re going to make Detroit proud — I promise you that.”

Contact Detroit Free Press music writer Brian McCollum: 313-223-4450 or bmccollum@freepress.com.