Turkish President Tayyip Erdogan and the Turkish government accuse Gulen of orchestrating Turkey's failed coup on July 15 in which more than 240 people were killed when rogue soldiers commandeered tanks, warplanes and helicopters, attacking parliament and attempting to overthrow the government.
Thousands of people turned up for the rally, which took place on Saturday, marking the beginning of the annual Newroz festival when Kurds mark the traditional New Year. "Turkey has tried on different levels to convince us of that fact, but they have not succeeded", foreign intelligence service chief Bruno Kahl told the German daily Der Spiegel.
The group is listed as a terror organisation not just by Turkey but also the European Union and the United States.
"How can terrorist organization be allowed to gather in Germany, while government ministers of the Republic of Turkey are not allowed to meet with our citizens?" he asked at a rally in the southern Osmaniye province's Kadirli district.
Erdogan's spokesman said the remarks were "proof" Berlin was siding with Gulen and his supporters. It's a sign of their support for FETO.
Police in Frankfurt, where hundreds of officers were deployed to the event, described the protest as peaceful and said on Twitter that most of the demonstrators had complied with German laws, adding: "We want to guarantee they can exercise their fundamental rights".
The ban on Turkish officials addressing rallies of expatriate Turks in various European countries, including the Netherlands and Germany, has brought relations between Ankara and the EU to a new low.
Tensions are already running high between Berlin and Ankara after German authorities refused to allow some Turkish ministers to campaign in the country for a "yes" vote in the April 16 referendum that would hand Erdogan an executive presidency. The Turkish government says the changes would boost stability and make governance more efficient. But Kahl also said he did not think the Turkish government was behind the coup.
Gulen, a former Erdogan ally who has lived in self-imposed exile in the U.S. since 1999, has denied the allegations.