Prosecutors said police launched a dawn raid on some 26 residences and other locations in Berlin.
"Based on information from abroad and international police investigations, there was reason to suspect that an Islamist of Arabic descent living in Berlin, as the head of a ... roughly 15-member group, was planning terrorist attacks in Russia," prosecutors said in a statement. They gave no details.
Prosecutors said the investigation was focused on three men aged 28, 30 and 36, but declined to identify them further. There was no word of any arrests.
They said police confiscated computers, various data storage devices and outdoor clothing in the searches, and that the material was being analyzed.