The gunman, who police said was a local resident, was killed after being "engaged" by officers who responded to report of multiple gunfire coming from the 32nd floor of the Mandalay Bay, a hotel-casino next to the concert venue.

Revelers screamed and fled in panic as a steady stream of automatic gunfire rang out at the venue shortly after 10:00pm local time, footage captured on smart phones showed.

"We have in excess of 100-plus injured and excess of 20-plus that have died at this point," Las Vegas Metro Police Sheriff Joseph Lombardo told an early morning press conference.

"This is an ongoing investigation, but we are comfortable that the primary aggressor in this event is -- has expired or passed away, and is no longer a threat."

Thousands of fans were attending the concert next to the Mandalay Bay which was part of a three-day country music festival known as Route 91.

Witnesses told how the gunman opened fire with an inital burst and then appeared to reload as he continued his spree. 

"We heard (what) sounded like a glass breaking, so you looked around to see what's going on and then heard a pop, pop, pop," Monique Dekerf told CNN.

"You'd think for a moment okay we're fine, there's no more gunfire, then it starts again."

Police officers point their weapons at a car driving down closed Tropicana Ave. near Las Vegas Boulevard after a reported mass shooting at a country music festival nearby on Oct. 2, 2017 in Las Vegas, Nevada. A gunman has opened fire on a music festival in Las Vegas, leaving at least 2 people dead. Police have confirmed that one suspect has been shot. The investigation is ongoing. (Agence France -Presse/Ethan Miller/Getty Images)

Her sister Rachel said it sounded like "the shots were coming from the right side ... it sounded like they were right beside us too ... it was right there."

Best-selling country singer Jason Aldean was on stage at the time of the shooting which came towards the end of the concert.

Second angle of the moment shots were fired. Video shows Jason Aldean running off stage following shots. #LasVegas pic.twitter.com/fnBe1tB1O7

— Kendall (78-84 💙⚾️) (@Seahawks_Fan95) October 2, 2017

"We were watching the concert having a great time, then we heard what sounded like firecrackers," witness Joe Pitz told the local Las Vegas Sun news outlet.

"I guess it was an automatic weapon going off but it literally sounded like firecrackers.

"Then soon enough there was commotion on the Mandalay Bay side of the stage. They were motioning for medics to come and safety people to come and Jason Aldean ran off the stage.

"Everybody in the vicinity went down. I don't know if they were ducking or if they were but it was chaos."