Emergency crews said wind gusts of between 60 and 70 miles per hour toppled the stage, where an estimated 12,000 people were waiting to see the band Sugarland on Saturday.
Forty people were hurt, some of them were critically injured.
One witness, David Wood, jumped in to help those who were trapped.
"It was chaos. I just got done running and pushing my wife out of the way of the stage that was coming down behind us. It was just kind of instinct to crawl back in and help the ones that were screaming that were in need," Wood said on "Good Morning America."
"People were crushed. They were just laying on the ground. Little girls being carried out because they were crushed from being in the front row," said witness Tori Iscon.
Indiana Lt. Gov. Becky Skillman opened the public memorial service at the fairgrounds' Free Stage with a prayer expressing thanks for those who rushed to help immediately after the stage collapsed.
Gov. Mitch Daniels and others sat in folding chairs on the stage lined with bouquets of sunflowers while many in the audience sobbed or hugged each other.
"I cannot tell you how proud I am," Daniels said.
The fair reopened Monday for the first time since the deadly crash.
Many people are asking questions about whether the tragedy could have been prevented. Indiana state police inspectors are looking at the rigging to see how well the stage was built. The stage builder is promising an investigation of their own. However, it could be weeks or even months before they have any answers.
The National Weather Service reports the wind burst at the time of the collapse was near hurricane strength.
Authorities said they were in the process of issuing an evacuation, but concertgoers said no one was told to leave.
The Associated Press contributed to this report.