Daphne Viola Webb vanished Wednesday evening when her father, John Anthony Webb, left her in the back seat of his sports utility vehicle while he went to buy an energy drink from a supermarket, the Oakland Tribune reported.
An elderly relative was in the front seat of the SUV at the time of the kidnapping.
Police said the girl was taken by an unidentified black or Hispanic woman 30 to 40 years old. Witnesses told police they saw the woman walk away from the car with the child.
Investigators have searched Webb's home, but have focused on Oakland's Martin Luther King Regional Shoreline Park, officials said. A police spokeswoman said Webb was not a suspect in his daughter's kidnapping.
Oakland police officer Johnna Watson said investigators have questioned other family members, including Daphne's mother, whose name was not released. The relationship between Webb and Daphne's mother was not known Wednesday evening.
The California Highway Patrol issued an Amber Alert for Daphne for Alameda, Contra Costa and San Francisco counties.