The fire was reported near Mission Inn Avenue and the Santa Ana river bottom, according to Riverside fire officials.
Mandatory evacuations were underway, but the locations were not immediately announced. An evacuation center was being set up at Dale Senior Center, located at 3936 Chestnut Street.
Large plumes of smoke were seen rising from the flames as winds up to 40 mph blew northeast.
DEVELOPING: We will add more details to this report as they become available.