Gunfire erupted at a gay nightclub in Orlando early on Sunday, causing mass casualties and leaving one suspected gunman dead inside, police said.
At least 20 people have been killed and over 40 injured in the shooting, according to reports.
Officers said in a Twitter message that there were "multiple injuries" at the Pulse nightclub and local media reported that between seven and 20 people had been shot.
The dance club urged patrons to "get out" and "keep running" in a post on its Facebook page.
Police have not confirmed if there were any other people killed or the specific number of injuries. A news conference was scheduled for 7 am ET.
Several patrons had posted on social media that a gunman was holed up inside and holding hostages. One man who said he was inside the club posted that the shooting broke out around 2 am and that he heard about 40 shots being fired.
Javer Antonetti, 53, told the Orlando Sentinel newspaper that he was near the back of the dance club when he heard the gunfire.
"There were so many (shots), at least 40," he said. "I saw two guys and it was constant, like 'pow, pow, pow,'."
Police said they had carried out a "controlled explosion" at the club hours after the shooting broke out, but did not say why that was done. They described the scene as a "fluid situation".
Video posted online showed a large number of police and emergency vehicles outside the nightclub. Bomb sniffing dogs were also on the scene, CNN reported.
It was the second deadly shooting at an Orlando night spot in as many nights. Late Friday a man thought to be a deranged fan fatally shot Christina Grimmie, a rising singing star and a former contestant on "The Voice", while she was signing autographs after a concert in the Florida city.