Its facade covered in murals and anti-capitalist graffiti, the occupied building at 94 Rigaer Strasse is among the squats that mushroomed across the city after the fall of the Berlin Wall.
The centre of "the Berlin anarchist scene" according to Germany's domestic security service, it has become the focus of a heated law-and-order debate flaring ahead of elections on Sunday.
While some want to see the counter-culture bastion wiped off the capital's map, others defend it as a vestige of an old Berlin rapidly disappearing as property prices and rents rise sharply.
Tensions flared in June when hundreds of riot police were sent to clear part of the building and evict its motley crew of inhabitants, sparking a battle that has raged on the streets and the courtrooms.
Located in grungy and hip Friedrichshain district, number 94, like other Berlin squats, is both a symbol and a victim of the city's huge transformation since German reunification a quarter century ago.
In the wild years after the 1989 fall of the Wall, which many Berliners remember with nostalgia, young people moved into abandoned buildings to party in techno raves, make art and experiment with free, alternative lifestyles.
Many squats were in working-class districts like Friedrichshain which, after the Wall fell, found themselves squarely in the centre of reunified Berlin.
Their Bohemian flair has since attracted a middle-class influx and driven up property prices, luring back absent landlords eager to turn crumbling buildings into multi-million-euro luxury condos.
'Orgy of violence'
Many former squats have morphed into legalised "alternative housing projects", with only the political posters on their facades bearing witness to their turbulent history.
But number 94 has pushed back against what many of its residents see as a police state serving greedy speculators.
Things escalated in June when riot police moved in to clear its ground-floor because the owner wanted the space back.
Some 300 officers ended up guarding the site - used as a communal kitchen and dining area - for three weeks to protect construction workers.
"Police were everywhere, we were asked for our IDs even when we were just walking down the corridors to go to the toilet," recalled one resident, speaking to AFP on condition of anonymity.
Bands of militants repeatedly reacted, class warfare-style, by roaming Berlin's streets at night to vandalise new buildings and randomly set fire to expensive cars.
On July 9th, some 3,500 protesters massed for what police described as Berlin's "most aggressive and violent demonstration in the past five years".
Dozens of people were arrested in clashes that saw demonstrators hurling stones, fireworks and bottles at the 1,000 police deployed for the protest.
Berlin's conservative minister in charge of security, Frank Henkel, called it an "orgy of violence".
Henkel - the lead candidate for Chancellor Angela Merkel's Christian Democrats in the city-state election - insisted he wouldn't "look the other way when cars are burning, there is violence and intimidation".
Campaigning for a "Strong Berlin", he warned that the city must "take left-wing violence as seriously as right-wing violence", which has flared since Germany took in one million migrants and refugees last year.
The residents of number 94, undeterred, challenged the eviction attempt in court, and scored a surprise victory when the judge found the owner had failed to issue proper notice ordering them to leave.
Police were ordered to pull out in a verdict that deeply embarrassed Henkel and emboldened the left-wing activists to file a separate suit against the "illegal police deployment".
Declaring they wouldn't budge, they vowed to "replace the weakened state apparatus with self-organisation" and pledged "appropriate measures against gentrification and the right-wing threat".
'Defend each other'
The building has become a cause celebre for Berlin's hard-left squatters and their sympathisers, including many people priced out of the booming housing market.
In a poll after the July clashes, 56 percent of Berliners surveyed said they favoured dialogue with the left-wing activists, while only 34 percent preferred tough police action.
The verdict was also cheered by activists squaring up for another eviction battle in neighbouring Kreuzberg district.
At the centre of it is Hans-Georg Lindenau, 57, who has run a "shop for revolutionary needs" since 1985, selling anarchist tracts and street battle gear ranging from pepper spray to helmets.
Previous owners failed to evict him, but the current one - the eighth, Lindenau said - has "more money, he has connections".
"Here, at Rigaer, everywhere... they are trying to make waves before the election to get votes," Lindenau told AFP, convinced he has been targeted "because of my political opinion".
In and near Rigaer Strasse, many neighbours also voiced support for their colourful neighbours.
One, who asked to be named only as Christof, said the violent demonstrations were "completely justified" because "it's through these actions that we defend each other".
A middle-aged man, who has lived in a now-legal squat at number 83 since the 1990s, argued that the police show of force sparked the violence and that both sides had overreacted.
Voicing fears that gentrification will kill the street's character, he confessed he had "divided" feelings.
"I'm not 20 anymore and I don't like finding a drunk man lying in my doorway, but I don't want this street to change," he said.
"There should be a place for different lifestyles."