When the storm hit, it knocked over trusses and the entrance collapsed on top of them.
'At approximately 5:30, the storm hit and knocked over the concert entrance trusses with around 70-80 mile an hour winds and heavy rain. Injured fans were treated on the scene and then taken to the hospital; two have since been released. When the outdoor venue was hit by the turbulent weather, the storm's intensity knocked over a metal entrance structure causing numerous show's attendees to be injured.
WinStar World Casino and Resort said that an evacuation began at about 5pm, when lightning was within four miles of the casino.
The casino said it asked fans to seek shelter from the incoming storm, but about 150 patrons decided not to.
Kevin Richardson, a member of the Backstreet Boys, tweeted about the situation and said that the status of the show is to be determined.
Richardson's band mate Nick Carter also tweeted that they were praying for the injured fans.
The concert by Backstreet Boy and 98 Degrees was canceled.
Fans were reportedly waiting in a large tented area at the WinStar World Casino and Resort in Thackerville when the harsh weather caused the awning to fall on top of them. "Status of show TBD".
He tweeted: 'Update... unfortunately, due to damage sustained in the storm earlier this evening, we are unable to perform the show tonight. The most important thing is for everyone to get home safely and we will see you all very soon!'
The Backstreet Boys, often referred to as BSB, formed in 1993 and rose to fame in the late 90s.
Organizers said the concert would be rescheduled and tickets to Saturday's show would be honored on the new date.