Protesters attend a demonstration against rent increase in Berlin, Germany, Saturday, April 6, 2019. Slogan in the foreground reads 'Stopp Deutsche Wohnen (housing company)' (AP Photo/Michael Sohn)

BERLIN (AP) — Affordable housing advocates in Berlin launched a grassroots campaign Saturday to force the city's government into taking over nearly 250,000 apartments worth billions from corporate owners to curb rising rents in one of Germany's hottest real estate markets.

The fight pits two philosophies against one another: free-market companies that see real estate as a means to profit, and housing activists who see low rents not only as a necessity but as central to the German capital's character.

"There need to be some rules here for the game — it's a city, not just open land for people to do what they want," said Thomas McGath, a representative of the group behind the campaign, known as Expropriate Deutsche Wohnen & Co. "It's not something that can be completely determined by the market."

The city had been a low-rent mecca after the fall of the Berlin Wall in 1989 opened the gates to the economically depressed former communist east of the city. That gave rise to an influx of artists and others seeking a more bohemian existence and prompted a former mayor to label it "poor, but sexy."

With the return of the government from the West German capital of Bonn, and its growing popularity as a high-tech and startup hub, the population has increased since 2004 from 3.39 million to 3.6 million.

Rents have been steadily rising at the same time, and even though the prices still lag behind other major German cities and European capitals, growing gentrification has been forcing many out of the very neighborhoods whose trendy vibe they created.

"We've had three landlords in the past seven years and the year before last we were informed our rents are going up by between 415 and 750 euros ($466 to $842) a month," said Paul Afred Kleinert, an author and translator who has lived in Berlin's gritty but now fashionable Kreuzberg district for 33 years.

Despite winning 15 court cases against their landlords in recent years, Kleinert said the pressure on tenants keeps increasing.

"There's a massive sell-off happening in this city," he said, adding that foreign real estate investors in particular seem to have few qualms about raising rents. "Entire streets have been purchased by Japanese, Norwegian or American consortiums," Kleinert said.

Still, according to the average monthly prices from the fourth quarter of 2018, 1,000 euros ($1,120) could get you an apartment of more than 80 square meters (860 square feet) in Berlin, but less than 60 square meters in Munich, Germany's most expensive city for rents.

The campaigning group's argument invokes Article 15 of the German Constitution, which provides for the "socialization" of "land, natural resources and means of production" by transferring it to public ownership. It's never been used before, but in principle should apply, said Christian Pestalozza, a constitutional and public law professor at Berlin's Free University.

"With Article 15, I would say the path is probably open," he said.

The group has introduced a resolution calling for the city to purchase the apartments of any corporation that owns more than 3,000 units, arguing the price should be "determined with due consideration of the interests of the general public" instead of market value.

The idea is they'd then be used as affordable homes administered by tenants and elected members of the public, and profits would be put back into maintaining and modernizing them, as well as constructing new housing.

The group's main target is Berlin's biggest landlord, Deutsche Wohnen, a publicly traded company that has 111,500 apartments in Berlin with an estimated market value of about 15.2 billion euros ($17 billion) — the majority of which were acquired from the city over the last decade or so as the capital struggled with debt. With those and the properties of several other companies, a total of about 243,000 apartments worth more than 36 billion euros ($40 billion) could be affected.

Deutsche Wohnen suggested the initiative was misguided, noting that its apartments average 60 square meters and rent for a modest 580 euros, and that even if they were sold to the city, it wouldn't solve the housing crisis.

"Expropriation is creating a lot of emotions right now, but it won't create a single apartment," CEO Michael Zahn told The Associated Press in an emailed statement. "An expropriation will bring no relief to the worryingly tight housing market in the capital."

Zahn added that already, half of rental income is reinvested in Deutsche Wohnen properties and 5% of new apartments are offered at lower rents or rent-free for "social purposes" such as housing for refugees or battered women looking to start new lives. He said the company also follows government rules used to calculate allowable rent increases by reflecting neighborhood averages.

The group behind the movement argues, however, that Deutsche Wohnen and others are so big that they can drive up an entire neighborhood's rent calculations, and perpetually justify further increases.

"There needs to be a limit at some point, because there are just too many big players coming into the market and completely screwing it up," McGath said.

Although radical, it's not just a pie-in-the-sky dream. As recently as 2014, activists were able to successfully use a referendum to block plans to develop a big part of the former Tempelhof airport and instead turned it into a massive park.

But just because it's possible, that doesn't mean it will happen anytime soon.

The first phase began Saturday with the group setting out to collect 20,000 signatures needed to bring its proposal to Berlin's government for consideration. The campaign kicked off with a rally for rent controls that drew thousands to Berlin's Alexanderplatz, a landmark square in the city's east, before marching to a real-estate convention. The protest was timed to coincide with similar demonstrations in other European cities.

McGath said he believes the necessary signatures will come together in a few weeks, at most.

If the government fails to come up with a proposal to the satisfaction of Expropriate Deutsche Wohnen & Co., the group will then need to collect 170,000 more signatures to trigger a referendum in the city-state of Berlin. Then it goes to a ballot, and a majority is needed to pass it.

Each step takes time, and there will inevitably be domestic legal action as well, Pestalozza said. Some have also suggested it could violate international laws regarding the expropriation of property.

"There could be a lot of conflict," he said. "It could be a decade until it's all said and done."

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Frank Jordans contributed to this report.

Protesters attend a demonstration against rent increase in Berlin, Germany, Saturday, April 6, 2019. Slogan in the foreground reads 'Living Is Not A Product - Stopp Rent Insanity'. (AP Photo/Michael Sohn)
Apartment buildings in the district Mitte photographed from the television tower in Berlin, Germany, Thursday, April 4, 2019. A campaign is being launched in the German capital to force the Berlin's state government into taking over nearly 250.000 apartments from corporate owners like Deutsche Wohnen and others. (AP Photo/Markus Schreiber)
Apartment buildings in the former east part of the German capital photographed from the television tower in Berlin, Germany, Thursday, April 4, 2019. A campaign is being launched in the German capital to force the Berlin's state government into taking over nearly 250.000 apartments from corporate owners like Deutsche Wohnen and others. (AP Photo/Markus Schreiber)
A protest banner reading : 'Therefore: Expropriate Deutsche Wohnen and Co' displayed at an apartment buildings in the district Kreuzberg in Berlin, Germany, Thursday, April 4, 2019. A campaign is being launched in the German capital to force the Berlin's state government into taking over nearly 250.000 apartments from corporate owners like Deutsche Wohnen and others. (AP Photo/Markus Schreiber)
Protesters attend a demonstration against rent increase in Berlin, Germany, Saturday, April 6, 2019. (AP Photo/Michael Sohn)