Fifty people died and another 53 were injured early Sunday when a heavily-armed gunman opened fire and seized hostages inside a club in Orlando

Fifty people died and another 53 were injured early Sunday when a heavily-armed gunman opened fire and seized hostages inside a club in Orlando.

"We have cleared the building, and it is with great sadness that I share we have not 20, but 50 casualties in addition to the shooter," Mayor Buddy Dyer told a mid-morning news briefing in Orlando, more than doubling the previously given death toll.

"There are another 53 that are hospitalized."

"It appears he was organized and well-prepared," local Police Chief John Mina said, adding that he had an assault-type weapon and a handgun.

Orlando authorities said they consider the violence an act of domestic terror. The FBI is involved.

The shooting began around 2 a.m., and an officer responded, Mina said. The officer engaged in a shootout outside the club. The gunman then 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. At that point police shot and killed the gunman, Mina said.

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.