Police were alerted shortly after 11:00 pm Thursday (0700 GMT Friday) when a female staff member reported that two armed men had entered the store, said LA Police Department (LAPD) spokesman Andy Neiman.
Officers scrambled and they surrounded the store, before calling in SWAT reinforcements, he said.
A little after 2:00am the SWAT team located 13 female staff and one male employee held in a storage room and bathroom, he said.
Some 200 people were also stranded inside a cinema at the shopping center during the drama.
"As we interviewed and debriefed all of the hostages, we learned that one of the hostages was stabbed," Neiman told Fox11 television. "That hostage has been treated for a non-life-threatening wound.
"Also, sadly, one of the other hostages was sexually assaulted," he added.
The incident comes amid intense debate in the United States about gun control, revived after a gunman killed 26 people including 20 young children at a school in Newtown, Connecticut in December.