The Rock am Ring festival was starting Friday in Nuerburg, but organizers suddenly announced police were advising they immediately clear out the grounds "due to a terrorist threat."

Fans appeared to be pouring out in an orderly fashion ... no stampede at all. In fact, thousands were singing along as they filed out of the Nuernburgring stadium. There was already a massive police presence -- 1,200 officers -- and backpacks had been banned at the event in the wake of the Manchester bombing.

Machine Gun Kelly, System of a Down, Macklemore, Liam Gallagher and Sum 41 are just a few of the acts scheduled to play the 3-day festival.

Police haven't released any specifics about the threat. Organizers are hoping to resume the concert on Saturday.