LAS VEGAS - Festivalgoers peacefully watching a country music concert near the Mandalay Bay Casino in Las Vegas initially didn't react to the staccato sound that drowned out the music.

Speaking to NBC Las Vegas, two concertgoers told The Independent they initially heard a noise "like firecrackers". But when it became apparent that a shooter or shooters were unloading "clip after clip" into the crowd, panic grew as people tried to get out of the area.

"It sounded like some fireworks going off," said the concertgoer, who did not give his name, in an interview with local television. "And then I think there was like the first kind of volley, then the second volley. My buddy was like "I just got hit - I got hit three times."

"Lots of people got hit," he added.

"It didn't sound normal, it sounded like machine guns and it was like several rounds, it was like hundreds of rounds," another witness told news station KSNV.

"My boyfriend had me move behind a building here because it just didn't sound right," she told KSNV. "And then we hid behind a building and we could just hear hundreds of rounds going off and then about 10 minutes later the police came and just blocked off all the streets."

A witness who spoke to News3LV said "bullets were flying everywhere" and concertgoers were running.

"They were firing from somewhere high and they were unloading clip after clip after clip after clip," the man said.

A female concertgoer added, "we all got on the floor, crawling out, rushing."

One man insisted there were "multiple shooters" involved.

Speaking to CNN, eyewitness Bryan Heifner, who was in a room in a hotel across from the Mandalay Bay, which he could see from his window, said: "Mostly I heard the shots, just so many shots - I just thought it was a semi braking with the air brakes, but then I went downstairs and saw people running and looking for family," he said.

"I immediately went back to my room, locked the door, turned the lights off."

Another eyewitness Ivetta Saldana told the Las Vegas Review-Journal that musician Jason Aldean stopped in the middle of his performance and was rushed off the stage. She was there with a friend and said the shots sounded like fireworks.

She said she hid in a sewer.

"It was a horror show," she said after getting out of the area. "People were standing around, then they hit the floor."

"The band ran off stage and it was pandemonium," Mr Jon Bessette, who was also at the concert, told NBC News. "Everyone was running, people were getting trampled."

The Mayor of Las Vegas tweeted: "Pray for Las Vegas. Thank you to all our first responders out there now."