From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
The Town Militia is a military group that is only mentioned and seen very briefly in a scene in Dragon Ball Z: The History of Trunks.
The Town Militia was a military faction, different from the one used by King Furry, that was present during many occasions in the anime. This group was apparently used as specific defense for just one city. After the Artificial Humans attacked Super World, they cut in line and killed security guards and rode on a roller coaster, leaving the man who was operating the roller coaster to call the Town Militia – not knowing that all of the soldiers had already been killed by the Artificial Humans. It is important to note that it is presumed that because Earth's Military was most likely completely destroyed by the Artificial Human that many cities created their own Town Militias, much like the one seen in this TV Special.