These groups of men were known by various names. Often in urban areas they were known as "night watchmen," as the period notice above indicates. They were also called patrollers, plantation police, home guards, or vigilance committees, but to the slaves they were simply the "pattyrollers." This semi-military organization, was often raised and supported by the local planters, but was recognized by the local and state authorities, and had permission to deal out punishments if necessary. They were often mounted for speed and exude and air of authority and were always armed. Duties of the slave patrols included arresting or discouraging runaways, monitoring the strict pass requirements for blacks traveling from plantation to plantation, breaking up large gatherings and assemblies of blacks, visiting and searching slave quarters at random, inflicting impromptu punishments, and if an occasion arose, suppressing insurrections.
The Civil War, President Lincoln's Emancipation Proclamation, and the Union occupation of much of the South, rendered these patrols doubly rigorous. Obviously, many blacks submitted to this rigorous searching reluctantly. The man in the picture below is none too pleased at being stopped and examined. He seems to unwillingly yield to the patroller out of no respect, but only by the potential threat his position and rifle afforded the patroller. The other slaves seem to show more of the stereotypical sycophantic attitude that was most often attributed to blacks in the mid 19th century.
Many slaves viewed the pattyrollers as merely an inconvenience that had to obliged. Some of the more bold slaves played tricks, or devised diversions for unsuspecting patrollers. Some booby trapped roads and trails to help discourage overly excessive vigilance on the part of the riders. One slave in an interview many years later commented that they would stretch strong grapevines across the road, and while one slave led the patrol down the road, the others would pull the grapevine taut and trip the horses, sending the riders flying.
After the Civil War the patrollers didn't go away in many areas. Groups such as the Ku Klux Klan and other terrorist groups developed and attempted to keep freedmen from organizing politically, meeting for schooling, or voting at the polls.
For more information on slave patrols check out Sally E. Hadden's excellent book, Slave Patrols: Law and Violence in Virginia and the Carolinas from Harvard University Press, 2001.