Lines of police protecting a pro-secessionist newspaper from a pro-Union mob, San Francisco, c. 1861.
"A rare photographic image of a nineteenth-century urban riot in progress. Sacramento Street east of Montgomery during the course of the disturbance, showing police lined up to thwart the intentions of those bent on destroying a ‘secesh’ newspaper. Charles and Michael De Young apparently scooped up printing and typesetting equipment left in the streets in the wake of riots against Confederate-sympathizing newspapers after Lincoln’s assassination, which they used to launch their newspaper The Daily Dramatic Chronicle." Link.
I’m not sure what year they they are implying by the caption but because of the “secesh” mention I think it is early in the war.