The incident happened at the Indiana State Fair just before a country music concert was to take place. High winds tore the blue tarp off the top of the music stage, which gave way shortly before 9 p.m. Eastern Time.
Four people have been killed, according to ABC affiliate WRTV. Several dozen others have been treated at area hospitals.
Indiana Task Force One along with several emergency crews already on-scene searched the debris of the stage after receiving reports of people trapped underneath. The search ended about two hours later.
Officials believe high winds, which reached up to 65 miles per hour at one point, was to blame for the collapse.
The country music band Sugarland was scheduled to perform shortly before the incident. The band tweeted their support to those who were injured Saturday.
"We are all right," the band said on Twitter. "We are praying for our fans and the people of Indianapolis."
Authorities said the Indiana State Fair would suspend activities on Sunday.