The scene is downtown Baltimore; the time is the afternoon of April 19, 1861. A week earlier, Fort Sumter had fallen to the Confederacy. Virginia was on the brink of secession from the Union. President Abraham Lincoln had called for troops to reinforce Washington, D.C.
At that time, Baltimore was the hub of the nation's railroad system (which, to an extent, it still is, as the Howard Street Tunnel fire proved). A municipal law forbade the operation of steam engines within the city limits. So when the 6th Massachusetts Volunteer Militia arrived at President Street station to the east of what is now called the Inner Harbor, they had transfer to horse-drawn cars to ride westwards across Pratt Street to the station at Camden Yards.
Despite having the nation's greatest population of free blacks, Baltimore was full of Confederate sympathizers and Maryland was a slave state. A mob gathered and barricaded the tracks; the soldiers got out of the cars and started marching instead. The crowd began pelting them with stones, oysters, and anything else handy. At Gay Street the mob erected another barricade, and some of the soldiers lost control and began firing into the crowd. All hell broke loose.
The best estimate has 16 killed (12 rioters, 4 soldiers) and more than 100 wounded in the initial confrontation. The soldiers made it to Camden Yards and steamed out of the city for Washington. Then the mob turned on the nearly 1000 soldiers from Massachusetts and Pennsylvania still waiting in a train at President Street. Pro-Union Baltimoreans turned out to join the fun. More casualties. Some soldiers hid with sympathizers in the city; others walked back to Pennsylvania.
As Fort Sumter had surrendered without casualties, the first blood of the American Civil War was spilled in Baltimore. By the end of May, both Baltimore and Annapolis were under Union control and kept under martial law, with habeas corpus suspended per Lincoln's orders. Marylanders fought on both sides of the war.
Some information taken from http://www.citypaper.com/2001-04-18/feature.html