A caller reported spotting the big cat around 9:30 p.m., and said she was 100 percent certain it was a mountain lion, police said.
There were no other reports of a mountain lion in the area and the animal has not been located.
Anyone who spots a mountain lion or has any information regarding Thursday's sighting should contact San Leandro police at (510) 577-2740.
People who encounter a mountain lion should not approach it, but should instead give the animal room to escape.
Do not run, but instead stand tall and face the animal and make eye contact.
People should do everything they can to appear larger, by raising their arms, opening their jackets or shirts, waving their arms and speaking firmly in a loud voice.
More information about mountain lions is available at www.keepmewild.org.
