An international network of hardline anarchist groups, including activists from Germany and Britain, has spent months planning attacks on property and violent confrontations with police in Genoa.
Calling themselves the Black Bloc, they regard the police as "guard dogs for the rich" and banks as legitimate targets for anti-capitalist actions. They wear black, one of the traditional colours of anarchism, along with masks to preserve anonymity from police cameras.
Aware of police and security service infiltration, they nonetheless communicate through Internet chat groups and websites. In Genoa, Black Bloc members persuaded local anarchists to guide them through the maze of medieval lanes, giving them an edge over police commanders not native to the city.
Police noticed anarchist missiles were often accompanied by insults in Genoese accents but after charging through the smoke to make arrests they found mostly foreigners.
The term "black bloc" was already in use when gangs ran amok during anti-globalisation protests outside the World Trade Organisation Meeting in 1999 in Seattle. Since then it has been applied to the highly organised groups who plan "actions" separately from mainstream protesters and have no contact with them or the police.
The majority of the black bloc in Genoa came from Germany, Italy, France and the Basque country in Spain. Few, if any, are believed to have been arrested. For more than two hours on Friday, and again on Saturday, groups of more than 100 people wearing black, burnt buildings, ransacked shops and smashed banks with crowbars and scaffolding. One group paraded with black flags and drums attacking cameramen and reporters, smashing their equipment and tearing up their notebooks.
Two months ago Mr Luca Casarini, leader of the Tute Bianche and Ya Basta groups, tried to meet black bloc representatives to persuade them to work non-violently. "But they refused to listen. We have no contact with them whatever," said Mr Casarini yesterday.
Among other anarchist groups and anti-globalisation protest movements, there is a suspicion about their origins. In Italy, the Green Party senator for Genoa, Mr Francesco Martone, alleged there was a history of collusion between the police and the neofascists to discredit the left: "There is evidence that they have worked together to both infiltrate the genuine protesters."
An Italian Communist MP, Mr Luigi Malabarba, yesterday alleged he had seen a large group of people dressed in black in one police station on Friday during the riot. "I saw groups of German and French people dressed as demonstrators in black with iron bars inside the police station near the Pizza di Kennedy. This is what I saw, draw your own conclusions," he said.
Video evidence collected by protesters and independent media suggests that men dressed in black were also seen getting out of police vans, effectively being taken to protest marches. They were noted for never attacking the police or the steel wall around the red zone of the city.
The Black Bloc insists it is part of the mainstream anti-capitalist movement. A statement by "participants of the Black Bloc" released in Genoa said: "We do not want to submit helplessly to the politics of the powerful. We have come to enter militantly the red zone and to stop the G8 meeting.
"Smashed windows of banks and multinational companies are symbolic actions. We do not agree with the destruction and looting of small shops and cars. However, do not let us be divided. To divide resistance is a usual way to weaken resistance."
The objective of a Black Bloc, as described on the main US website, is to "provide solidarity in the face of a repressive police state and to convey an anarchist critique of whatever is being protested that day."
Its origins may go back to the earlier German Autonomist movement, one of the anarchist coalitions which emerged from the wreckage of the Baader-Meinhof group. Another German group which advocated confrontation in Genoa was FAU, or the Free Workers' Union, an anarcho-syndicalist trades union.
FAU rejects Germany's social market arrangements. It is more interested in direct action.
Its website declares: "We don't just talk. We also do."