Video posted on social media appeared to show the moment the gunfire broke out during a song, stopping the performance and sending the crowd running.
A gunman, named as 64-year-old Nevada resident Stephen Paddock, opened fire from the 32nd floor of the Mandalay Bay Hotel towards an open-air music festival attended by 22,000.
Police say he killed himself as officers stormed the room where 10 guns were found.
The attack is the worst mass shooting in recent US history.
US President Donald Trump tweeted his “warmest condolences and sympathies” to the victims and their families, and called the shooting “terrible”.