Thanks for your effort. However, I agree with Ciphergoth that this particular tactic is likely to result in a loss of reputation in those circles that may actually impact x-risks - as a result, I think that it will indirectly increase net x-risk. I feel somewhat awkward advising against this because you've already made a public effort towards organizing it, but I am pretty sure that it will have a negative impact.
Consider that one of the odd quirks of human psychology is that people often consider the primary argument made in favor of a proposition to be the best argument for that proposition - namely, they often assume that you've made the best argument that you can. However, this setting will not enable you to make a very good argument at all - you can't overcome very large gaps of inferential distance with 3-4 words on a sign. Even people who might otherwise be receptive to the importance of x-risks will remember "we saw some people waving signs around on the street about that a few days ago" - they'll be less receptive than they would have been otherwise to more rigorous arguments because you've placed yourself in the same reference class as people with much-less-defensible positions.
I appreciate the effort that you've put into this - the whole "get people working together to achieve something" is arguably something that we could substantially improve at as a community. However, this is a case where some well-intended actions will have a negative impact compared to no action at all. (Keep in mind, though, that "no action at all" isn't the relevant counterfactual - what else could you do with the time to generate fuzzies and/or utilons?)
A lot of people went to streets to protest against nuclear weapons which were considered existential risks, and it resulted in nuclear disarmament in 1980. See photo here: http://www.atomicarchive.com/History/hbomb/page_18.shtml
Any important topic has street actions, Greenpeace, PETA, you name it. If you don't have street actions it means that no body is interested in the problem.
Existential risks are the risks of human extinction. A global catastrophe will happen most likely because of the new technologies such as biotech, nanotech, and AI, along with several other risks: runaway global warming, and nuclear war. Sir Martin Rees estimates these risks to have a fifty percent probability in the 21st century.
We must raise the awareness of impending doom and make the first ever street action against the possibility of human extinction. Our efforts could help to prevent these global catastrophes from taking place. I suggest we meet in Union square, San Francisco, September 27, 2014 at 2:00 PM in order to make a short and intense photo session with the following slogans:
Stop Existential Risks!
No Human Extinction!
AI must be Friendly!
No Doomsday Weapons!
Ebola must die!
Prevent Global Catastrophe!
These slogans will be printed in advance, but more banners are welcome. I have previous experience with organizing actions for immortality and funding of life extension near Googleplex, the White house in DC, and Burning Man, and I know this street action, taking place on September 27th, is both legal and a fun way to express our points of view.
Organized by Alexey Turchin and Longevity Party.
Update: Photos from the action.