'Mass casualties' in Florida gay club attack, shooter dead: Police

An unidentified gunman opened fire inside a crowded gay nightclub in Florida early today, wounding an undetermined number of people in what police described as a "mass casualty" event.

Police said the shooter was dead, four hours after the incident began with a volley of gunfire in the Pulse nightclub in downtown Orlando, triggering a massive deployment of police and emergency vehicles.

"We can confirm this is a mass casualty situation," the Orlando police said on their Twitter account.

"The shooter inside the club is dead," they said in a separate tweet. It was unclear whether there were any fatalities among the club patrons, with police due to brief the media at 7 a.m. local time.

It was the second shooting incident in the city in just over 24 hours, coming shortly after singer Christina Grimmie was shot dead late Friday by a gunman who stormed a theatre where she had just finished a gig.

Witnesses said that the shooting erupted at around 2:00 a.m. amid throbbing music as the crowded club was nearing closing time.

Speaking to Sky News, clubber Ricardo Negron, who was inside when the shooting began, described how the gunman raked the club with bullets.

"People just dropped on the floor. I guess the shooter was shooting at the ceiling because you could see all the glass from the lamps falling," he told the network.

He described hearing "non-stop firing" which probably lasted less than a minute but felt like a lot longer.

"There was a brief pause in the shooting and some of us just got up and ran out the back," he said.

"People have definitely been injured — or worse," he added.

Emergency vehicles swarmed the area, with at least one crisis command vehicle at the scene.

"Shooting at Pulse Nightclub on S Orange. Multiple injuries. Stay away from area," Orlando police wrote on Twitter.

"There are injuries. I am not sure if there are any deceased at this time," a police dispatcher told AFP, adding, "Officers are going in to search the building and to get people out."

The club advertises itself online as "Orlando's hottest gay bar".

On its Facebook page, the club warned patrons: "Everyone get out Pulse and keep running."

Witness Christopher Hanson said he heard "loud banging noises, like gunshots going off".