German police have raided hundreds of addresses in an operation targeting suspected Islamic militants.
German police confiscated items during the raids
The searches were aimed at suspected members of the Caliphate State group, one of a number of groups banned after the 11 September attacks in the US.
The group's leader's flat in Cologne was apparently one of those searched.
German Interior Minister Otto Schily said in a statement that the searches had prevented the organisation carrying on its activities.
More than 5,000 police took part in the dawn raids in 13 of Germany's 16 states, searching apartments and seizing evidence.
The federal prosecutor's office said the targets included the residences of four people suspected of planning attacks on as yet unspecified interests.
The Caliphate State group calls for establishing an Islamic state in Turkey and is considered by the German Government to be potentially violent and a threat to security and social peace.
Mr Schily has described the group as a "breeding ground for Islamic terrorists".
He said the raids were launched because Caliphate State members were violating a government order from December 2001 to disband, and the measures taken would be a clear warning to other Islamic extremists in the country.
The Caliphate State was set up in 1984 by Metin Kaplan of Turkey, who finished a four-year prison term in March for inciting group members to murder a rival Islamic leader.
German authorities want to extradite Metin Kaplan to Turkey
Germany is currently trying to extradite Kaplan to Turkey, where he is wanted on treason charges.
Mr Kaplan's Cologne flat was among those searched, according to the AFP news agency.
The German Federal Criminal Office, Germany's equivalent of the FBI, has carried out a number of raids against Islamist groups this year.