The attack happened Sunday, on the last night of the Route 91 Harvest Festival, a three-day country music event. Online video showed the singer Jason Aldean, who was listed as the festival’s final act, performing as the shooting began.
Early Monday, Mr. Aldean posted a photo of the words “Pray for Las Vegas” over the city’s skyline. The attack “hurts my heart,” he wrote.
Tonight has been beyond horrific. I still dont know what to say but wanted to let everyone know that Me and my Crew are safe. My Thoughts and prayers go out to everyone involved tonight. It hurts my heart that this would happen to anyone who was just coming out to enjoy what should have been a fun night. #heartbroken #stopthehate
Jake Owen, a country singer who had performed earlier in the night, said on CNN on Monday that he was onstage with Mr. Aldean when the attack unfolded.
“It’s like shooting fish in a barrel from where he was,” Mr. Owen said, referring to the gunman, who the police said carried out the attack from a hotel room at the Mandalay Bay Resort and Casino. “I feel so bad for these people. We’re so lucky that none of us were in the line of fire.”
Mr. Owen added that the shooting continued for some time.
“This is not an exaggeration: This shooting was going on for at least 10 minutes,” he said. “It was nonstop.”
Several other performers included in the festival lineup also offered sympathy online.
The singer Luke Combs posted a photo on Instagram of himself onstage earlier on Sunday, along with a pointed message.
“I wanted to post a photo of what this festival was supposed to be about and not give this horrible person the recognition he so desperately wanted,” Mr. Combs wrote. “I am incredibly saddened by tonight’s events and while I’m thankful that me and all of my band and crew are alive and unharmed, I can’t help but hurt for all the people who weren’t as fortunate and the pain their loved ones must feel.”
The musician Josh Abbott said that his group, the Josh Abbott Band, had been evacuated from the Mandalay Bay resort.
“It was a long, awful night but we are blessed to be alive and healthy,” he said in a statement. “Hug your loved ones tight.”
In a Twitter post, Kane Brown, a country singer who had performed hours before the Las Vegas attack, according to the schedule, offered prayers for Las Vegas. In another, he issued a four-word reaction: “This world is sick.”
The pop singer Ariana Grande, whose concert in Manchester, England, was struck in May by a suicide bomber who killed 22 people and wounded hundreds more, wrote on Twitter, “My heart is breaking for Las Vegas,” and called for more gun control measures.
Britney Spears, whose show is running at Planet Hollywood in Las Vegas, said on Twitter that she was “completely heartbroken” by the attack. Jennifer Lopez and Celine Dion, who are also performing in the city, echoed that sentiment.