Katy buys fans a drink

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Katy Perry

Katy Perry thrilled fans at a hometown concert in Santa Barbara, Calif., on Saturday by buying them all a drink.

The pop star was closing out her Witness: Coming Home benefit show to aid wildfire and mudslide relief efforts in the area at the Santa Barbara Bowl, when she offered to treat the entire audience to a drink.

Fans immediately turned their backs on the headliner and headed for the bar, prompting Katy to urge them to stay and catch the rest of her show first.

The Firework singer told Entertainment Tonight at the American Idol semi-finals on Sunday that it was impressive how quickly she managed to clear out the whole audience with her offer.

"Last night I was doing a show in Santa Barbara for the mudslide relief in Montecito, and there's a lot of guys who are there 'cause I think they want to be, but I know that they're usually there to support their wives or girlfriends or kids," she explained. "I'm like, 'You know what? I'm opening up the bar and buying everyone a beer!' So all the men - even my own sister's husband - left and they all came back double fisting (holding two drinks). It was amazing."

Santa Barbara Bowl general manager Rick Boller told TMZ after the gig that the venue's drinking spots were suddenly awash with fans claiming their free booze - and Katy agreed to pay up.

The American Idol judge told ET she doesn't know how much the offer will cost her, joking that the venue owners are "still crunching the numbers."

Her fellow judge Luke Bryan added, "She told them to hold the check till Friday," to which Katy joked, "Clear that check!"

The season finale of American Idol takes place on Monday.