German authorities have shut down a popular rock music festival because of a possible terrorist threat.

The dpa news agency reported Friday that the Rock am Ring festival was cleared out on its opening night after Koblenz police said they'd received information of a "concrete threat."

Organizers posted on the festival website that "due to a terrorist threat the police have advised us to interrupt the festival."

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It wasn't immediately clear how many fans were in attendance, but some 90,000 were expected by the end of the weekend at the annual three-day event outside the western town of Nuerburg.

The German band Rammstein was the Friday night headliner, but hadn't yet started playing.

Festival organizers say they hope to be able to go ahead with the program Saturday.