Four men, aged from 28 to 37 years old, were arrested on suspicion of preparing a serious act of violent subversion.
Around 50 officers raided four apartments in the Schöneberg area of the capital early on Wednesday morning.
The men arrested are all Turkish citizens. They are suspected of gathering equipment for Isis in Syria, including night vision goggles and binoculars.
The investigation is continuing.
In Stuttgart, meanwhile, 24-year-old Ismail I. admitted to a court on Wednesday that he had returned to Germany from Syria to buy supplies for Isis fighters.
Ismail is on trial with two accomplices.
His lawyer said that Ismail had had a “troubled past“ up until his arrest, describing him as a lost soul given to earthly pleasures before his encounter with fundamentalist Islam.
After a failed marriage, Ismail was promised that “if you believe in Allah, everything will be better,“ at a mosque in Stuttgart.
His unhappiness made Ismail "easy prey" for fundamentalists when he was sent on a pilgrimage to Mecca, the security services (Verfassungsschutz) reported.
After travelling through Turkey, Ismail joined Isis fighters in Syria in August 2013. He was trained in using weapons and stood sentry for the group, as well as being involved in house-to-house fighting in the northern city of Aleppo.
Quickly realizing that he wasn't cut out for battle, he was sent back to Germany in October 2013 to buy equipment, including night vision goggles, for the fighters.
But he was caught when a hunting supplies shop owner became suspicious and called the authorities.
Ismail's lawyer is hoping his client's confession will earn him a suspended sentence.
Germany officially made support for Isis illegal in September.
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