At least two people have died after a gunman opened fire on crowds at an outdoor music festival in Las Vegas.
Multiple people have been injured in the mass shooting near the Mandalay Bay Hotel and Casino on the Nevada city's famous gaming strip.
Footage posted on social media appeared to show people fleeing machine gun fire at Route 91 Harvest country music festival.
Police said "one suspect is down" and they did not believe there were any other gunmen.
Two festival-goers reporting hearing a noise "like firecrackers" before realising a shooter was unloading "clip after clip" into the crowd.
Others reported seeing muzzle flashes from the upper floors of the Mandalay Bay Hotel and Casino across Las Vegas Boulevard towards festival.
A hospital spokesman said at least two people had died and 24 more were injured.
"Confirming that one suspect is down," tweeted Las Vegas Metropolitan Police Department. "This is an active investigation. Again, please do not head down to the Strip at this time."
They later added: "At this time we do not believe there are any more shooters."
Police have placed the area on lockdown and closed off parts of the Las Vegas strip and Interstate 15.
Some officers took cover behind their vehicles while others carrying assault rifles ran into the hotel.
Reports on social media, citing police scanners, said there was a gunman on the 32nd floor of the hotel who had shot at a bodyguard and police.
Twenty-six people were admitted to University Medical Centre, spokeswoman Danita Cohen said. At least two died and 12 were in critical condition, she added.
Las Vegas Metropolitan Police Department told people to avoid the area and have cordoned off several roads.
The department posted on Twitter urging people to not to "livestream or share tactical positions of our officers on scene."
"This may put emergency responders in danger," it added.
Las Vegas City Hall tweeted: "Please avoid the south end of the Strip. Very sad night for Las Vegas."
Witnesses said country singer Jason Aldean was playing near the end of the festival when gunfire rang out.
Kodiak Yazzie, 36, said he and his girlfriend were watching the performance when he heard what sounded like fireworks. The music stopped temporarily and started up again before another round of pops sent the performers ducking for cover and fleeing the stage.
Thousands fled as bursts of gunfire could be heard for more than five minutes, Mr Yazzie said.
Witnesses said they saw multiple victims and dozens of ambulances near the concert venue. Some attendees later huddled in the basement of the nearby Tropicana hotel-casino.
McCarran International Airport, about two miles away, grounded all planes and diverted some incoming flights after reports of the shootings emerged.
