Country singer Jason Aldean was performing during the Route 91 Harvest concert outside of Mandalay Bay on the Las Vegas Strip on Sunday night when someone opened fire on the crowd, causing people to duck for cover and run for safety in between rounds of automatic gunfire.
The gunfire was coming from the window of a room on the 32nd floor of the Mandalay Bay hotel, police said.
Concertgoers had to climb over barricades set up for the event to get to safety.
Las Vegas police were quickly dispatched to the area.
Part of the Las Vegas Strip was shut down and the Mandalay Bay hotel was put on lockdown.
Las Vegas PD Sheriff Joe Lombardo said there were more than 50 fatalities and more than 400 people wounded.
Concertgoers administered medical aid to victims until they could be loaded into ambulances and other vehicles for transport to hospitals.
Police said the gunman was located in his hotel room but they believed he killed himself before they arrived.