Survivors of the worst mass shooting in American history are describing a gruesome scene of chaos at the downtown gay club Pulse.

Christopher Hansen says the gunfire stretched several minutes."All I heard was, bang bang bang bang bang! And when I came out of the bar it was like, get back-get back-get back", Hanson remembered.

Hansen says he tried to give blood after the shooting early Sunday morning but was turned away because the Food and Drug Administration limits donations from gay men.

Authorities say the shooter, identified as Omar Mateen, eventually retreated to a bathroom where club-goers were hiding. They breached a wall using explosives and an armoured vehicle.

When Mateen came out firing officers gunned him down. Fifty-three were wounded in the massacre, the worst mass shooting in American history.