| Last year's May Day in Berlin was the most violent in a decade [GALLO/GETTY]
Lukas sits drinking a beer in Baiz bar, a popular hangout for Berlin's leftwing scene.
He has a steady job, is well-groomed and wears a brand-name jacket. There is nothing that readily identifies him as a leftist radical.
Yet the 25-year-old is explaining what members of Germany's far-left jokingly refer to as "sports groups" - teams of young men whose "sport" consists of beating up neo-Nazis.
"A friend calls and says, 'There are neo-Nazis down in the park, playing their music'," he says, describing one incident he was involved in. "We said, 'Not anymore.' It was the first and last time they were there."
Lukas was in many such brawls in his younger days. Well-built and trained in martial arts, he survived them unscathed, though a friend once caught a broken bottle in the face and still has a thick scar to prove it.
Although he has retired from the "sport" and moved into more organised political activity - which is why he does not want his full name used - Lukas says he is still perfectly comfortable justifying violence against neo-Nazis.
"A friend once told me, 'You have to speak the language that people understand. Those guys, the neo-Nazis, they don't understand any other language'."
'Declaration of war'
Violence, it seems, is increasingly the preferred language of Germany's estimated 6,300 leftwing extremists.
The country, which is still grappling with its Nazi history 65 years after the fall of the Third Reich, has been shaken in recent months by a spate of brazen attacks targeting police, government buildings, large corporations and personal property - notably the almost nightly arson attacks on expensive cars.
|In 2009, there were more leftwing than rightwing crimes in Germany [GALLO/GETTY]
Last month, Germany's interior ministry announced that there were 9,375 leftwing crimes committed in 2009 - a startling 39.4 per cent rise on the previous year.
Violent crime, including arson, rose even more sharply, jumping 53.4 per cent to a total of 1,822 offences.
For the first time since the current system of record keeping began in 2001, assaults committed by the left outnumbered those by the right - 849 against 800 - most of which were directed either at police during rallies or at neo-Nazis.
More than 400 cars - mostly expensive ones - were torched in Berlin and Hamburg last year.
In December, about 10 masked attackers set upon a manned police station in Hamburg, setting a patrol car on fire and hurling stones through the windows - an incident the head of the police union branded a "declaration of war".
With the traditional leftwing day of protest, May 1, on Saturday, authorities are bracing themselves.
Last year's May Day in Berlin was the most violent in a decade, with hundreds of arrests and dozens of police officers injured.
Yet the causes of this surge in crime and violence remain frustratingly opaque.
Florian Herbs, 26, an unemployed graphic designer and member of the radical group Antifascist Revolutionary Action Berlin (Arab), admits that car burning has become a fashionable "cult".
But beneath that, there is genuine anger about the gentrification of previously poorer, working-class and immigrant districts of Berlin.
|Arson has become a popular tool of Germany's leftwing extremists [GALLO/GETTY]
Wealthier residents are moving into leftist and anarchist heartlands such as Kreuzberg and Friedrichshain, pushing up rents and fuelling the clandestine attacks on cars, offices and upmarket property developments.
The backlash, Herbs says, is working: Dozens of attacks on property developments have served as a real deterrent to gentrification, he argues.
"Now nobody wants to buy one of those apartments."
To Lukas, the former "sportsman", the question is: Why shouldn't the left use violence?
"Why should we be the only ones to refrain?" he asks.
"It's too easy to say, 'Violence is terrible'. But this society is full of violence. The difference is that we don't go around beating immigrants."
A new generation
So what do they want? Experts say leftwing German militants today are very different from their predecessors.
Issues such as nuclear weapons and even the more recent anti-globalisation movement have faded in importance.
"The leftwing extremist scene is made up of a very heterogeneous group of people with different ideological views," says Stefan Ruppert, an expert on extremism and an MP for the pro-business Free Democrats party.
"Only the vague goal of overthrowing our existing social order serves as a unifying effect. This complexity ... makes it all the more difficult to grasp the problem as a whole and work out solutions."
His assessment is borne out by Herbs, who says his Arab group, which has 30 to 40 members, is one of about 30 radical groups in the Kreuzberg district alone.
Neighbouring Friedrichshain, another leftwing stronghold, probably has another 30 groups.
Further complicating things, some neo-Nazis - so-called nationalist autonomists, who answer to no political organisation - have come to resemble the radical left "autonomists" in appearance and even share their anti-capitalist views.
"That increases the potential for violence during confrontations between left and right wing extremist groups, which in turn raises the danger for the police forces," Ruppert says.
Legitimising each other
Indeed, some analysts point out that, without each other, the two sides would not have much to do.
"They legitimise each other," said Viola Neu, an expert on political extremism at the Konrad Adenauer Foundation.
|Leftwingers blockaded a far-right march in Dresden in February [GALLO/GETTY]
"They are there because the other exists. They are extremely interested in what the other lot is doing. They watch each other, they fight, and their arguments go back and forth."
No one is sure what to expect on Saturday.
Rightwing extremists are planning their own march through the leftist district of Prenzlauerberg - a gesture many on the left see as a provocation.
What is more, the left is full of confidence in the wake of its success in blockading a far-right march in February commemorating the 1945 allied bombing of Dresden. It will try the same on Saturday, forcing the police to intervene.
"Maybe there will be clashes," says Herbs.
"Maybe some people will start throwing things. There is a dynamic to the moment that we can't control."
"We hope it will stay peaceful, but with the capitalist crisis and gentrification, people are very angry. We'll see what happens."