Known for disrupting major international meetings across Europe for three decades, the far-left, black-clad anarchists are loosely organised with vague political demands but share a set of violent tactics.
Germany's Office for the Protection of the Constitution said in its annual report released this week that black blocs typically seek "direct confrontation with the political opponent or with the police".
"They try to provoke the police with violence and by igniting fireworks or throwing bottles and rocks to bring about an escalation," it said.
"That happens in the hope that the 'repressive capitalist state' will be 'unmasked' with its reaction to this militancy."
And that's the case in Hamburg, where radical leftist demonstrators have laid siege to the city centre while Chancellor Angela Merkel convenes the leaders of the world's top economies.
In running street battles, a hard core of around 1,000 militants have ignored authorities' demands to remove their masks - which are illegal at German demonstrations - as riot police move in repeatedly with water cannon trucks and tear gas.
Even before the talks on terrorism, climate change and trade could begin, the 'black bloc' claimed a propaganda victory with images that resembled an urban war zone captured by the world's media.
Led by small commando units, the black blocs serve as provocateurs from within larger, often peaceful demonstrations, with young Germans, Italians and Spaniards among the most frequently detained. Some travel from event to event.
While leaders themselves gather in venues protected by a ring of steel, citizens of the host cities often bear the brunt of the riots, just as in Hamburg where dozens of cars parked on local streets were torched and shopfronts vandalised.
The German police officers' union GdP accused them of "hijacking peaceful demonstrations by tens of thousands of people to deliberately attack" authorities.
From squats to summits
The shadowy groups, given the name black blocs by police, got their start in West German cities such as Hamburg and Berlin in the 1980s, and university towns such as Goettingen and Freiburg.
Initially focused on fighting local battles with police over squatter evictions and nuclear power, the targets in subsequent decades shifted to geopolitics, such as the 1987 visit by US President Ronald Reagan to West Berlin.
After the Berlin Wall fell, May Day workers' holidays were repeatedly marred by pitched battles in German cities between far-leftists, resurgent neo-Nazis and police.
In 1999, black blocs appeared on the fringes of massive anti-globalisation protests during the World Trade Organization ministerial conference in the US city of Seattle.
The next year saw violent street fighting between anarchists and riot police during the annual assembly of the International Monetary Fund and the World Bank in Prague, and a European Union summit in the French city of Nice.
And in 2001, protesters using similar tactics clashed with police during the G8 summit in Genoa, northern Italy. The provocations sparked a police crackdown leading to the shooting death of one protester.
Eight years later, black bloc demonstrators armed with iron bars infiltrated a peaceful demonstration outside a NATO summit in Strasbourg France and caused chaos outside a climate summit in Copenhagen.
The rioters had pushed many international meetings in recent years to seek out rural, hard-to-reach venues. However Merkel opted to host the G20 in Germany's second city this year for logistical reasons, given the size of the delegations, and as a symbol of transparency.