Arrest warrants were issued for 27 police officers in an operation across 13 provinces and focused on Istanbul.

Turkish police detained 18 people including senior police officers on Tuesday, in an investigation of alleged supporters within the police force of President Tayyip Erdogan’s foe, Muslim cleric Fethullah Gulen, state-run Anadolu Agency said.

It said arrest warrants were issued for 27 police officers in an operation across 13 provinces and focused on Istanbul. The private Dogan news agency said the operation focused on former officers, including police chiefs.

Operations targeting alleged supporters of Gulen have continued unabated since a parliamentary election won convincingly on Nov. 1 by the AK Party which Erdogan founded.

Last month dozens of people, including senior police officers and bureaucrats, were detained in an operation in the western city of Izmir over alleged links to what prosecutors have dubbed the “Gulenist terror group”.

Gulen was formerly an ally of Erdogan and believed to have wielded considerable influence in the judiciary and bureaucracy.

Erdogan turned against the cleric and his followers, accusing them of constituting a “parallel state”, after police and prosecutors seen as sympathetic to Gulen opened a corruption investigation into Erdogan’s inner circle in 2013.

The cleric has lived in self-imposed exile in the United States since 1999 and is the subject of arrest warrants in Turkey. A prosecutor is seeking a prison sentence of up to 34 years on allegations that he sought to topple Erdogan. Gulen denies the allegation.

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