Authorities informed the management of the website linksunten.indymedia.org on Friday that they were shutting it down, DPA has learned from sources inside the ministry.
Security sources claim that the website is the most influential platform for the militant left, and that it is used as a forum by extremists willing to use violence.
Users can post comments anonymously on the site, which was started in 2009. The militant left often post responsibility for political crimes on its pages, such as what happened when anti-G20 activists published a letter of responsibility for attacks on Germany's train network in June.
Security sources claim that the website played an important role during the riots at the G20 in Hamburg in July, with people calling for violent protest on its forums.
At the G20 protests in Hamburg in July, groups of black block anarchists burned cars, smashed in windows and looted shops in several neighbourhoods of Hamburg during several days of rioting. Authorities say that 500 police officers were injured during the violence, while the government created a €40 million fund for victims of the damage.
Justifying the move to shut down the website, the interior ministry said that it “goes directly against the law in both its aims and actions”, according to Spiegel, which first reported the story.
Police in Freiburg, Baden-Württemberg also searched properties linked to the the site's management.
“At the moment several premises are being searched,” said Thomas Strobl, interior minister in Baden-Württemberg on Friday morning. “This step marks a major blow against the extreme left in Germany.”
No one is reported to have been arrested in connection with the investigation, but computers and other evidence have been seized.
The most recent report by the BfV, Germany's internal spy agency, described linksunten.indymedia.org as “the most important platform for violent left-wing extremism in Germany. For years it has been providing a forum for people to publish first-hand reports on left-wing crimes.”