Critics have called the picture racist, but Grose claims he was unaware of the racial stereotype that blacks like watermelon.
Grose had planned to resign at the city's council meeting Monday night.
City officials expect a large turnout at Monday's meeting. The Captain of the Los Alamitos Police Department says there will be a beefed-up police presence.
Grose won the council seat in a 2006 election, and was chosen by the council for a largely ceremonial one year term as mayor in January.
Grose said, "I recognize that I've made a mistake and have taken steps to make sure this is never repeated."
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