Roseville Fire Inspector Tom Dodaro tells KXTV that the spontaneous holiday musical performance drew a huge crowd.
Officials say there were reports of the floor and foundation moving and creaking near the food court, which spurred the evacuation.
Flash mobs are groups of singers or dancers whose surprise performances in public places are often recorded and posted online.
The group Monday was there to sing Handel's "Hallelujah Chorus" in an event organized by the Sacramento Choral Society and Orchestra.
The evacuation was orderly, and the singers planned to sing outside the mall.