At least five people have been killed and dozens injured after an outdoor stage collapsed in windy weather at the Indiana State Fair.

Strong winds caused the stage rigging for the outdoor concert to collapse on Saturday night, trapping and injuring concert-goers.

Indiana State police spokesman David Bursten told the AP news agency that a "strong gust of wind upset the rigging above the stage at the Indiana State Fairgrounds and caused a collapse of the structure over the stage".

Emergency crews were called to the scene, and workers were setting up a command centre to tend to those who were injured.

The collapse came as thousands of concert-goers were being evacuated from the fairground grandstand to a nearby coliseum.

The first act had completed and Sugarland, the Grammy-winning country duo, was about to begin its performance.

"It was like it was in slow motion,'' Amy Weathers told the Indianapolis Star. "You couldn't believe it was actually happening.''

Sugarland released a statement on its website, expressing its sadness at the incident.

"We are all right after our stage collapse at the Indiana State Fair tonight,'' the statement said.

"Many of our fans and friends in Indianapolis may not be. Please keep them in your thoughts, prayers, or whatever form of strength you are able to send."

Source: Agencies