Some people started to scatter and search for cover.
But some people stayed behind - or even made their way to the chaotic and deadly scene - to help the victims of the deadliest mass shooting in modern American history.
A gunman had opened fire as country music singer Jason Aldean sang onstage.
At least 50 people were killed, including an off-duty police officer with the Las Vegas Metropolitan Police Department, authorities said. More than 400 others, including two on-duty officers, were injured.
The gunman, identified by police as 64-year-old Stephen Paddock, was found dead in his hotel room on the 32nd floor of the Mandalay Bay Resort and Casino by Las Vegas SWAT officers.
Former minor league baseball player Todd Blyleven, who had travelled with family and friends from Dallas for the concert, said they were centre stage, toward the back, when they heard a "pop-pop-pop."
"You look up and you could see the muzzle flashes from the window at the Mandalay Bay," he told The Washington Post. "The stage went black, Jason Aldean and his crew all ducked and ran offstage. Everybody started screaming, and then you start to see people going down."
Blyleven said he and his brother-in-law guided their group out of the venue, ducking behind barbecue vendor carts, as gunshots continued to strike around them. Once he had gotten his group out, Blyleven headed back into the concert venue, joining a group of volunteers who hoped to get more people out.
"The shooting just seemed like it went on for 10 or 15 minutes," said Blyleven. He said he saw a police officer who had been shot through the neck, and carried out the lifeless body of a young woman who had been hit.
"Young girls and guys, older folks. Just people walking out of a country concert with bullet holes," he recalled. "Everybody was just trying to do whatever they could to get these poor people out of the gunfire."
Blyleven, who is the son of Hall of Fame pitcher Bert Blyleven, said he has no formal medical training but that he felt obligated to do whatever he could to save lives.
"I just felt like I had to," said Blyleven, who estimated that he may have helped about 30 or 40 people get away from the gunfire. "I would hope that if me, or my family, was in a situation like that, that someone would come in and get me."
During the gunfire, Mike McGarry, a 53-year-old financial adviser from Philadelphia, said he tried to shield his children.
"It was crazy - I laid on top of the kids. They're 20. I'm 53. I lived a good life," McGarry told Reuters. He said he had shoe prints on the back of his shirt from people who ran over him to get away.
A parade of police officers, firefighters and paramedics rushed to the scene of the shooting, where good Samaritans were seen in photos kneeling down, tending to victims.
One man told Fox News that he hid behind a table and, when it was all over, helped load several bodies into a truck.
One video showed someone approach a vehicle, telling the driver: "Hey Bud, right now, we need your truck. We just need to get people over to the hospital, OK?"
"OK," a woman said from inside the vehicle. "Put them all in the back."
In the background, people appeared to be carrying victims on pieces of fencing to get them to safety.
"Oh my God," the woman said, her voice shaking. "Baby, this is horrible."
In the hours after the shooting, a nearby blood donation centre became full, and people who lived nearby were trying to find ways to help those still at the scene.
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Jessica Perez, who was at home in bed when she heard about what had happened, told the Las Vegas Review-Journal that she loaded water into her car and she and her brother and her cousin headed for the scene of the shooting.
"We couldn't just sit at home doing nothing," the 21-year-old from Las Vegas told the newspaper. "Everyone was begging us, please don't go, but we couldn't sit there."
"My heart hurts, and I can't believe this is happening in my home," Perez added.
