What is the mass demonstration about?

In newspaper articles, radio and television announcements and even mobile phone messages, Spaniards are encouraging one another to take part in a planned mass demonstralion this evening to protest against terror and yesterday’s bloody attacks. Across Spain, people held candlelit vigils throughout the night, expressing their sympathy with the numerous victims. The Spanish government has called for three days of national mourning. Meanwhile the official investigation and public speculation into who was behind the atrocity continues.