Fethullah Gulen: Erdogan ally turned public enemy number one
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The 80-year-old, who has lived in self-imposed exile since 1999, denies any link to the attempted overthrow of his erstwhile ally Erdogan.
Rather, Gulen insists he heads a network of charitable organisations and businesses named "Hizmet" (Turkish for "service").
Since the failed coup, Turkey has tracked down suspected members of Gulen's network abroad and claims to have "repatriated" dozens of individuals.
German federal prosecutors on Friday said they arrested a Turkish national on September 17 on suspicion of spying on the Gulen movement for Turkey's secret services.
Gulen and Erdogan were once close, with the Turkish president profiting from the Muslim preacher's network to cement his power after he first became prime minister in 2003.
But Gulen became the Turkish strongman's "public enemy number one" after a corruption scandal towards the end of 2013 which touched the inner circle around Erdogan, who was prime minister at the time.
Erdogan accuses Gulen of leading a "parallel state" which sought to oust the president, but this is a claim that "Gulenists", supporters of the preacher, deny.
Since the July 2016 failed putsch, more than 300,000 have been arrested in Turkey over suspected ties to Gulen and nearly 3,000 have been sentenced to life in prison.
And during an unprecedented purge, over 100,000 public sector workers were sacked or suspended, including around 23,000 military personnel and 4,000 judges.
The Turkish secret service has also carried out several operations in central Asia, Africa and the Balkans to forcibly bring back alleged Gulen supporters.
In July, Ankara said it had "repatriated" a Turkish teacher living in Kyrgyzstan, Orhan Inandi, who had gone missing a few weeks earlier and whom the Turkish authorities presented as a member of the Gulen movement.
Turkey has also sought unsuccessfully to extradite Gulen from the US and pressured other countries, especially in the Balkans, central Asia and Africa, to close Gulen-linked schools.
His movement was previously influential in the Turkish media, police and judiciary, and is sometimes described as a "sect".
The members "help each other in business, they have a missionary mentality and a grand sense of enterprise," said Sam Brannan, from the Center for Strategic and International Studies (CSIS) think tank, in 2014.
Supporters are expected to give money or their time to the movement, from students to housewives to wealthy businessmen.
But Turkish officials say Gulen manages a "terrorist" group which they refer to as the "FETO terrorist organisation" and whose members are closely supervised by "mentors" in their careers and their private lives.
The purge which followed the failed coup targeted Gulen network members but also pro-Kurdish officials and journalists in what critics described as an opportunistic crackdown on anyone who opposed Erdogan.
Ankara insists the purge is a bid to remove the "virus" of Gulen from the state.