ORLANDO: A gunman opened fire at a crowded, well-known central Florida nightclub, and multiple people have been wounded, police said Sunday.

The shooter inside the club is dead, Orlando Police said on the department's official Twitter page. It was not immediately clear how the shooter died, or how many people were wounded in the shooting. Police described it as a "mass casualty situation."

Police reported "multiple injuries" following the shooting at Pulse Orlando, a popular gay dance club. People were told to stay away from the area.

Dozens of police vehicles, including a SWAT team, swarmed the area around the club. At least two police pickup trucks were seen taking what appeared to be shooting victims to the Orlando Regional Medical Center. Police said via Twitter that a noise heard near the shooting site was a "controlled explosion." No other details were provided.

Pulse Orlando posted on its own Facebook page around 2 a.m.: "Everyone get out of pulse and keep running." Just before 6 a.m., the club posted an update: "As soon as we have any information we will update everyone. Please keep everyone in your prayers as we work through this tragic event. Thank you for your thoughts and love."

Police said local, state and federal agencies were investigating.

The incident follows the fatal shooting late Friday of 22-year-old singer Christina Grimmie, who was killed after her concert in Orlando by a 27-year-old Florida man who later killed himself. Grimmie was a YouTube sensation and former contestant on "The Voice."

Jon Alamo said he was at the back of one of the club's rooms when a man holding a weapon came into the front of the room.

"I heard 20, 40, 50 shots," Alamo said. "The music stopped."

Club-goer Rob Rick said it happened around, 2 a.m., just before closing time.

"Everybody was drinking their last sip," he said.

He estimated more than 100 people were still inside when he heard shots, got on the ground and crawled toward a DJ booth. A bouncer knocked down a partition between the club area and an area in the back where only workers are allowed. People inside were able to then escape through the back of the club.

Christopher Hansen said he was in the VIP lounge when he started hearing gunshots.

"I was thinking, are you kidding me? So I just dropped down. I just said please, please, please, I want to make it out," he said. "And when I did, I saw people shot. I saw blood. You hope and pray you don't get shot."

He continued to hear shooting even after he emerged, where police were telling people to back away from the club. He saw injured people being tended to across the street.

Mina Justice was outside the club trying to contact her 30-year-old son Eddie, who texted her when the shooting happened and asked her to call police. He told her he ran into a bathroom with other club patrons to hide. He then texted her: "He's coming."

"The next text said: 'He has us, and he's in here with us,'" she said. "That was the last conversation."