UPDATE: At least 50 confirmed dead and 42 injured in Florida club shooting
At least 50 people are lying dead inside Pulse, a gay nightclub, after a gunman stormed in and opened fire, Orlando Police have confirmed.
Chief John Mina said at least 42 people have been transported for medical treatment
Police have shot and killed the gunman, Mina told reporters.
Police sources have identified the gunman in the mass shooting as 29-year-old Omar Mateen, an American citizen whose parents are from Afghanistan, according to US media reports.
"It's appears he was organised and well-prepared," the chief said, adding that the shooter had an assault-type weapon, a handgun and "some type of (other) device on him."
Law enforcement sources told CNN that the device, which was strapped to the suspect, was possibly explosive, but authorities don't know if it was real or not.
A canine unit indicated there were explosives inside the suspect's car as well, the sources said.
At least nine officers were involved in the shootout, Mina said. Several coroner vans were seen traveling toward the nightclub.
One officer suffered an eye injury when a bullet struck his Kevlar helmet, said Danny Banks, special agent in charge of the Florida Department of Law Enforcement's Orlando bureau. The helmet saved the officer's life, Banks said.
The shooter is not from the Orlando area, Mina added.
Orlando authorities said they consider the violence an act of domestic terror. The FBI is involved. While investigators are exploring all angles, they "have suggestions the individual has leanings towards (Islamic terrorism), but right now we can't say definitely," said Ron Hopper, assistant special agent in charge of the FBI's Orlando bureau.
The shooting began around 2am, and an officer who was working at the club responded, Mina said. The officer engaged in a shootout outside the club, after which the gunman ran into the club.
"That turned into a hostage situation," Mina said.
Authorities were getting calls from people inside the club but away from the gunman, the chief said.
More police rushed to the scene and broke down a door with an armored vehicle, helping some 30 club-goers flee to safety, Mina said. At that point police shot and killed the gunman, he said.
CNN affiliate WKMG video captured clubgoers carrying injured people from the club.
"It's just shocking," said Christopher Hansen who was inside Pulse. He heard gunshots, "just one after another after another. It could have lasted a whole song," he said.
"Everyone get out of Pulse and keep running," the nightclub posted on its Facebook page shortly after the violence began.
Orlando police warned residents to stay away from the area, and urged people not to call their offices. They declined to provide the number of casualties, saying details will be available later. Bomb sniffing dogs are on the scene.
Pulse describes itself as "the hottest gay bar" in the heart of Orlando. Hours before the shooting, the club urged party-goers to attend its "Latin flavor" event Saturday night.
Jovial, well-dressed crowds heeded the call in an event that turned into a nightmare.
"It was just, bang, bang, bang!" party-goer Hansen said of the gunfire.
Ricardo Negron Almodovar said he was in the club when the shooting started about 2 a.m. He barely escaped.
"People on the dance floor and bar got down on the floor and some of us who were near the bar and back exit managed to go out through the outdoor area and just ran," he posted on the club's Facebook page.
"I am safely home and hoping everyone gets home safely as well."