Orlando Gay Nightclub Shooting -- 50 Dead ... Biggest Toll in American History

A gunman opened fire inside a gay nightclub early Sunday morning in Orlando -- injuring multiple club goers, before engaging in a gun battle with police.

50 people are dead an 50 more wounded. Police also say the shooter is dead inside Pulse Nightclub.

"Everyone get out of Pulse and keep running" ... was posted on the club's Facebook page shortly after the shooting began. Videos and photos flooded social media, several showed a standoff -- apparently between cops and the shooter.

The mass shooting happened 24 hours after singer Christina Grimmie was murdered In Orlando ... during a fan meet and greet following her concert.