Protest? 3.13.04

This afternoon in front of the U.N. there will be a protest against the terrorists (whoever they are. We don't yet know) who blew up the train in Madrid.

Who are these people protesting? What is a protest going to do? Someone who likes to protest, please tell me what effect it has. Registers outrage? I think most human beings understand the outrage (though the Daily News cover calling the attack Spain's September 11th is a bit much. It's their March 11th); and is there something that could have been reasonably done to prevent this? Was there a warning that was ignored? Have the politicians (Spainiards are getting set to elect a Prime Minister) somehow encouraged this kind of activity? Was there slack vigilance? No? Then what is this protest going to consist of?