More than 50 people have been killed and at least 200 injured in a mass shooting at a Las Vegas music festival, police said.

An off-duty police officer was among the dead after a gunman opened fire on crowds at the Route 91 Harvest country music festival on the Nevada's city's famous strip.

Dozens of police vehicles descended on the strip after authorities received reports of an active gunman.

Las Vegas police later killed a suspect on 32nd floor of a hotel next to the concert. They said they did not believe there were any other attackers.

Thousands of people fled after the "lone wolf" shooter sprayed bullets at a crowd watching country star Jason Aldean.

Festival-goers reported seeing muzzle flashes from the upper floors of the Mandalay Bay Hotel and Casino, across Las Vegas Boulevard from the music festival.

They heard the sound of what they described as automatic gunfire.

People run for cover at the Route 91 Harvest country music festival (Getty Images)

A woman who was attending the concert with her husband told CNN: “Everyone was literally lying on top of each other and trying to get out of the way, and the shots just kept coming.

“When we got down, there was a man shot right there. One of my friends was helping him out. They got him out, and we ran, and everybody is hiding everywhere. They were hiding under the bleachers [seats], and the stanchions, and anywhere they could.

“And everybody is telling us to run, ‘run as fast as you can’.

“My husband and I ran out towards our car, and there were people hiding underneath my car for cover.

“There was a gentleman who was shot, and he said ‘can you help me’? So I put him in my car. I had, like, six people in my car.”

The suspect was a Las Vegas man who acted alone, said Sheriff Joseph Lombardo of the Las Vegas Metropolitan Police Department. But authorities were seeking an Asian female roommate of the suspect as person of interest, he added.

The Las Vegas Metropolitan Police Department said Marilou Danley was "sought for questioning" over the shooting and urged anyone who saw her to call 911.

Police were also looking for two vehicles.