An unidentified gunman opened fire inside a crowded gay nightclub in Florida early Sunday, 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 am (1100 GMT).

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 theater where she had just finished a gig.

Witnesses said the shooting erupted at around 2:00 am (0600 GMT) 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."

"People have definitely been injured -- or worse."

- 'Everyone get out' -

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. "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."

"I didn't see any of the actual shooters. I just saw bodies going down and I was ordering a drink at the bar.

"I fell down. I crawled out. People were trying to escape out the back. I just know that when I hit the ground, I was crawling and I hit my elbows and my knees," he told CNN.

He said there was "blood everywhere."

Rosie Feba was at the club with a friend when the shooting broke out.

"She told me someone was shooting. Everyone was getting on the floor," Feba told the Orlando Sentinel. "I told her I didn't think it was real, I thought it was just part of the music, until I saw fire coming out of his gun."

The incident came on the heels of a deadly shooting at the nearby Plaza Live Theater in which a heavily-armed gunman shot and critically wounded singer Grimmie, a former contestant on the popular TV talent show "The Voice."

She was rushed to hospital but died of her wounds.

The theater is located less than four miles from Pulse nightclub.

The Plaza gunman, 27-year-old Kevin James Loibl, then killed himself.

He was found to be armed with armed with two handguns, ammunition and a hunting knife, police said. His motive remains unknown.