Just ask Kotatsu, they knew a whole week beforehand that something was going to go down, and just a tweet was enough set off the protests in motion, all organized by political parties, NGOs and such.
*facepalm* Wow. Just...Wow.
Yes, Twitter, facebook, and so on can be used to pull together demonstrations in a hurry--in fact, it takes very
little to organize a demonstration quickly. You pick a time and a place, and then...
Tell people about it. That's really all there is to it. And considering that text messages, twitter, facebook, and so on make telling people about it super-easy, that really takes a lot of the work out of it.
Posters? Signs? Most people make their own, and it doesn't take a long time to grab a piece of posterboard and make up a small sign. Others, such as activists who go to a lot of demonstrations, quite likely already have pre-made signs regarding the subject that can be re-used.
There's really no
need to plan a "response demonstration" months in advance. The only kind of rallies that absolutely
require that sort of planning are the ones where you expect thousands of participants, like the 2003-04 anti-war rallies. In those cases, you need to plan for permits, street closings, port-a-potties, first aid, and so on. But just to get some people out on (public) street corners with signs? No significant planning needed.
I can tell you for a
fact that at least here in Atlanta there was no advance preparation for a demonstration, but one was called together as soon as people heard about the raid.
No, there wasn't some international conspiracy to organize a response demonstration before the fact.