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.
He killed himself as police stormed the room where 10 guns were found.
Investigators have found no link to international terrorism, despite a claim from so-called Islamic State.
In an address from the White House, President Donald Trump described the attack as "pure evil".
