While the civilians are large, they are not powerful. The Civilians start off with few units in their cities and the units are quite weak.
The first goal for the civilian player should be to consolidate their forces by building at least 4 more civilian units for each city. After that, the civilians will have to stockpile their Neighborhood watches (about 40 to 50 or more depending on skill level) would be a good army for the civilians to root out the crime in the city. As with most of the players in the game, the civilians should focus on a single civilization, such as the Crips or Bloods and destroy them one by one. It would be a waste of your weak forces to confront all of the criminals at once, especially because they will have a multitude of barracades (city walls) protecting their city.
If you have rooted out the criminals, you have won the game, as the police are not a target you should pursue (or can persue for that matter.)