There are few existing anonymous protesting networks at present, especially one site which combine anonymous and protest all together.
Protest.NET might be a good place to find where those upcoming protest event about to happen, however, it doesn’t provide discussion nor the security for some certain events. You can find out what when and where event is going to happen without struggle, easy but brings concerns for protesters in some other countries – local authorities do whatever it takes to prevent such thing happen.
Facebook and Twitter might be good place to call out for a protest, considering the large population base they have. But again, sensitive materials would be removed or banned at very beginning. So they won’t be the first choice either.
WHYWEPROTEST is quite a mature protest network, but only focusing on protesting Scientology. They do have forums to discuss other events take place internationally, but users are confused at the very beginning from entering the site, which looks more like a designed Scientology protesting site rather than to wider topics. Also the anonymity of the site is very weak – email address is required when registers and no compulsory to use secure/anonymous connection.