Police swarmed on one of Las Vegas's most famous casinos after reports of an "active shooter" there.
Panicked guests at the Bellagio hotel said they were forced to flee and hide amid claims of a shooting.
Guests have shared dramatic photos and videos of the crowds gathering outside the famous strip in Las Vegas.
Las Vegas Metropolitan Police Department later confirmed there were no injuries, and that reports of an active shooter were false.
Officers said they are investigating a burglary, which reportedly took place at the Rolex store inside the hotel.
One woman caught up in the incident tweeted: "Mass panic at Bellagio.
"Everyone running, I'm currently ducked on the floor under a table."
Some witnesses said they saw a man armed with a gun near the Rolex watch shop.
Images emerged online that appear to show the suspect wearing a pig mask.
A large police presence can be seen in footage posted on Twitter.
The Bellagio resort contains almost 4,000 hotel rooms in two separate buildings.
It is famed for its grand fountains and lake, alongside a casino, restaurants, art gallery and botanical gardens.