More from Kevin Rutherford, Billboard
The "Healing in the Heartland" concert, which aired live Wednesday on NBC, also included performances from Reba McEntire, Usher and Luke Bryan, among others.
Authorities reportedly arrived on the scene and ordered the gathering crowd to disperse or be arrested for trespassing.
The event will be open to the public and held at Los Angeles' Hollywood Palladium. Hanneman died May 2 of liver failure.