See the nice pictures on visualresistance.org and click to the previous posts at the bottom of the page.
See the blog for more Visual Resistance and Street Art Workers posters.
If you want to try it yourself, read How to make street art, but read the Don’t Get Caught!-part; "If you get caught, you might want to try to repeat this mantra –”art project”– and hope it works. If not make sure you ask: “Are you detaining me? Am I under arrest? I want to assert my right to remain silent! I want to speak to a lawyer!”