I am not a big fan of demonstrations or petitions; they often seem to me to be more mob rule than serious deliberation. For the 1,000 that show up for a demonstration, were there 10,000 that had a different opinion? For the 100,000 who signed a petition, were there 1,000,000 who had a different opinion? Just like all the no-shows on voting or the uncalled on polling, we can never be sure what the inactive think.
However, like letters to the editor, one should often raise one's voice to at least make an opinion known.
Reading today's Al Jazeera blog on Libya I saw that Avaaz has created a petition urging the UN Security Council to impose a no-fly zone in Libya. Avaaz's campaign for sanctions two weeks ago with 450,000 emails stunned the Security Council office.
One must admit, such actions often do get attention.