Revelers screamed and fled in panic as a steady stream of automatic gunfire rang out at the venue
Police said the gunman, a 64-year-old local resident named as Stephen Paddock, had been killed after a SWAT team responded to reports of multiple gunfire 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 10:00 pm local time (0500 GMT Monday), footage captured on smart phones showed.
"We are looking at in excess of 50 individuals dead and of 200 individuals injured at this point," Las Vegas Metro Police Sheriff Joseph Lombardo told a pre-dawn press conference on Monday in the Nevada gambling hub.
"Obviously this is a tragic incident and one that we have never experienced."
Lombardo said that police and FBI were still looking into Paddock's background but they had "located numerous firearms within the room that he occupied" in the hotel.
Police said that Paddock had opened fire on the crowds below from the 32nd floor of the giant hotel located on the famous Las Vegas Strip.
Paddock's female companion, who had earlier been named as a person of interest by police, is believed to have been located, Lombardo added.
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.
Donald Trump was briefed on the "horrific tragedy", the White House said, and the US president took to Twitter to offer his "warmest condolences and sympathies to the victims and families."
- 'Like firecrackers' -
Witnesses told how Paddock opened fire with an initial long 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.
Las Vegas shooting
"You'd think for a moment okay we're fine, there's no more gunfire, then it starts again."
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 and near the end of his concert when the shooting began.
"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."
- 'Beyond horrific' -
Although the final toll has yet to be confirmed, it is already the deadliest shooting in the United States.
The previous deadliest shooting came in June 2016 when 49 people were killed at the Pulse nightclub in Orlando, Florida.
It was also the latest in a series of recent deadly attacks at concert venues.
Twenty-two people were killed while leaving a concert in the northern English city of Manchester in May when a suicide bomber detonated a nail bomb in the foyer.
Ninety people were killed in November 2015 at the Bataclan venue in Paris during a concert by the US band the Eagles of Death Metal.
A shocked Aldean told his fans via Instagram that he and his band was safe.
"Tonight has been beyond horrific," the singer wrote.
"I still don't know what to say ... 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."