Authorities have warned people to stay away from the area as the situation continues to unfold, The Dallas Morning News reported.
Initial witness accounts suggested that at least seven people had been shot at Pulse club.
Accounts further indicated that a WESH-TV reporter has said the assailant may have shot 20 or more people and had barricaded himself in the club.
The violence reportedly erupted before 2 a.m.
Police have confirmed that they responded to a shooting at the nightclub, and numerous emergency vehicles were responding. Officers on the scene were armed with assault rifles.
However, no details of the incident have been confirmed, and information remains sketchy.
An Orlando newspaper reporter apparently has indicated that the gunman also may have been armed with a bomb.
One person who said online that he had been at the club when the shooting began reported that he managed to escape because he was close to an exit. Other people tried to take cover by dropping to the floor.