About 20 tents belonging to festival-goers at the Green Man festival, near Crickhowell, have been raided by thieves.
Police tweeted that the tents were targeted in a 24-hour period as the festival got under way at the Glanusk Estate.
A man has been arrested and is in police custody, they added.
Grammy-nominated Ryan Adams and two-time Mercury prize-winner PJ Harvey are among the acts headlining at the four-day event.
PJ Harvey's headline set will be broadcast live on 6 Music on Sunday from 10pm.
Police have previously warned motorists to expect extra traffic around Crickhowell and the Brecon Beacons area this weekend.