AUSTIN, Tex. — A few hours before a gunman shot up an outdoor concert in Las Vegas, horrifying the nation, Tyler Costolo bought tickets for another big music extravaganza — Austin City Limits Music Festival, the annual celebration that kicked off Friday afternoon.

Despite the Las Vegas massacre, which killed 58 and left hundreds injured, Mr. Costolo, 25, a water-polo coach, stuck with his plans. He came to Texas from his home in Boca Raton, Fla., and on Friday morning, he was at the entrance to Zilker Park, determined not to let the tragedy in Nevada diminish his love of music.

“I’m kind of the opinion things like that shouldn’t change your life,” said Mr. Costolo, wearing a white T-shirt with the name of the band the Front Bottoms. “At that point, you’re letting those kinds of things win.”

Austin City Limits is the first major music festival since the Las Vegas shooting. Part rock ’n’ roll extravaganza (a headliner this year is Jay-Z) and part arts festival, it draws thousands of attendees to its sound stages, snack stalls and a kid-friendly area dubbed Austin Kiddie Limits, complete with diaper-changing stations. The first bands took the stage Friday.

On Friday afternoon, among music lovers and families with strollers flowing into Zilker Park on the shores of the Colorado River, the mood was partly defiant and partly alert and attentive. But mostly, people were unflappable.

Under an autumn blue Texas sky, thousands of people milled — past the mobile charging station, past the Wine Lounge — many shedding their sandals to walk barefoot in the grass. Children darted ahead of their parents, performing cartwheels.

Couples spread out on blankets or watched from folding chairs. And a broad assortment of musical groups, from the swing band Asleep at the Wheel to new-wave pop bands, performed on more than a half-dozen stages scattered in the park.

Artists and spectators alike acknowledged that Las Vegas cast a pall over the festival. Organizers, assisted by the Austin police, bolstered security, patting down fans and conducting searches as people neared the grounds.

“Get ready to take everything out of your bag,” a security assistant shouted as spectators approached the entrance.