Clearly, the underlying causes of the protest which has spread to other city squares around the country are the economic crisis and the government's austerity measures that have unemployment in Spain to 21 percent. For young people, the figure may be twice as high.
But the trigger seems to have been outrage at levels of political corruption, with a widely distributed map showing towns and districts where more than 1,000 politicians are standing for office in the weekend municipal elections despite being the subject of official investigation. They include Francisco Camps, the powerful head of the regional government of Valencia, and many other targeted in the so-called Guertel case, a far-reaching inquiry into bribes and kickbacks alleged to total some $170 million.
Even more telling is the way Spanish protesters complain at what they call the 'Berlusconi-zation' of their country. It is at once a reference to the clownish behavior of Italian Prime Minister Silvio Berlusconi and his apparent immunity from legal sanction and a protest at the emergence of a privileged political class in Spain that seems equally capable of avoiding punishment for scandals.
Across Europe, political figures have not only been seen to be behaving badly but have usually got away with it. This helps explain the fascination with the case of Dominique Strauss-Kahn, who resigned last week from his post as head of the International Monetary Fund after being arrested and charged with the attempted rape of a hotel maid in New York. The image of a powerful man, widely tipped to become the next President of France, being seen in handcuffs and prison overalls was a potent reminder that the law is meant to work for the weak and the powerful alike.
In Anglo-Saxon societies, justice usually works. In Britain, former Environment Minister Elliot Morley was sent to prison for 16 months Friday for fiddling about $45,000 from his parliamentary expenses. Three other former MPs have been imprisoned for similar offenses. In the United States, President Bill Clinton faced impeachment over allegedly lying to avoid a sex scandal.
But despite the publicity over his "Bunga-Bunga" parties with naked call-girls, some of them below legal age, and his phone calls to get at least one such girl released from police custody, Berlusconi seems untouchable. His governing majority has changed laws and gained time for the statute of limitations to end his court cases over tax avoidance and graft.
In Germany, where the law also seems to function as it should, the political class has been discredited by a different kind of scandal, the academic offense of plagiarizing in order to win a university doctorate.
This year, the popular Minister of Defense Karl Theodor zu Guttenberg, was forced to resign his post after he was found to have taken large sections of his Bayreuth University doctoral thesis from other people's work, including newspaper articles, without having sourced the information.
The most interesting feature of this affair is that he was exposed not by university authorities or political opponents but by a virtual non-governmental organization called VroniPlag. The group publishes online, printing the evidence from both the original thesis and the sources from which it seems to have been copied.
"The goal is to guarantee the integrity of doctor titles in Germany," VroniPlag says on its Web site. "Our work is neither politically motivated nor is it aimed at personal defamation or anything like that."
VroniPlag has since taken on new targets, most prominently the vice president of the European Parliament, Silvana Koch-Mehrin. From the German Free Democrat party, she is a member of the European Parliament and is accused of "copying" about a quarter of her 10 year-old doctoral thesis on currency reform. Veronica Sass, the lawyer daughter of the former Bavarian Prime Minister Edmund Stoiber, is under investigation by her old university after VroniPlag claimed that part of her thesis on telecoms regulation had been taken from a Wikipedia article.
Bribery and corruption, attempted rape, sex with under-age girls, cheating on expenses and cheating on their academic credentials -- the record of Europe's political class would be unsavory at the best of times but in the context of the economic crisis that has sent unemployment and budget deficits soaring, the European public is reacting angrily.
And modern technology means they have the means to do, with Spain's squatting protesters in the campsite on the Plaza del Sol using Facebook and Twitter to rally support just like their predecessors in Cairo's Tahrir Square. Germany's VroniPlag uses computer programs devised to prevent academic cheating in the United States to identify plagiarists in high places.
There is something international about this phenomenon, with the Spanish protesters claiming Cairo as their inspiration and using President Barack Obama's "Yes, we can" slogan for their own motto, displayed around their makeshifts tents. It reads "Yes, we camp."