HUNDREDS of Turkish police officers have been sacked and some moved to traffic duties, further undermining a fraud investigation Prime Minister Tayyip Erdogan portrays as a covert attempt by a rival to usurp state power.

Mr Erdogan, facing the biggest challenge of a 10-year rule that has seen the military banished from politics, the economy booming and Ankara pressing its influence in the Middle East, portrays the operation as a "dirty plot" by followers of a US-based Islamic cleric.

Fethullah Gulen, who does does not back any political party, enjoys broad influence in the police and judiciary.

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The Turkish government has hit back by sacking or reassigning hundreds of police across the country since the investigation began last month, while a second investigation into large infrastructure projects championed by Mr Erdogan has been blocked.

Around 350 officers in Ankara, including members of the financial and organised crime, smuggling and anti-terrorism units, have now been dismissed or reassigned to new roles including traffic or district duties.

Prosecutors, meanwhile, have deepened their investigations, with at least 25 more people including public officials detained as part of an investigation into the activities of a port in the Aegean province of Izmir.

Timothy Ash, head of emerging markets research at Standard Bank, said: "Neither side appears willing to give up at this stage in this high-stakes battle for control of the state."

Mr Erdogan and the Hizmet movement of cleric Fethullah Gulen, which exercises influence through a network of contacts built on sponsorship of schools and other social and media organisations, accuse each other of manipulating the police and compromising the judiciary's independence.