Katy Perry Buys Drinks for Santa Barbara Crowd During Benefit Concert

Katy Perry treated her hometown to free booze at a concert this weekend -- but might've spoken too soon ... 'cause much of the crowd went runnin'.

Katy performed Saturday night at the Santa Barbara Bowl for what she dubbed 'Witness: Coming Home' -- a benefit show to help relief efforts for the wildfires and mudslides in the area earlier this year. All ticket sales were donated, including Katy's cut.

Toward the end of the performance, Katy offered to buy the entire audience "all drinks" on her dime, per eyewitnesses. True to form ... many in the SB crowd immediately hit the bar.

You can hear Katy try to call them back in to catch the rest of the show, but it was too late. Santa Barbara Bowl's general manager, Rick Boller, tells TMZ that her offer amounted to about one free drink per person to anyone who swung by. No word yet on the total tab.