Turkish Prime Minister Recep Tayyip Erdogan’s government purged hundreds of police officers overnight, news media reported, as part of a crackdown on a rival he accuses of trying to usurp state power by means of a specious corruption investigation.

Some officers, including members of the financial-crime, organized-crime, smuggling and anti-terrorism units, were moved to traffic duties, according to the reports.

Despite the dismissals and reassignments, police and prosecutors continued to carry out arrests, targeting the state railway company and a western port Tuesday.

Erdogan, who is facing the biggest challenge of his 11-year-old tenure — during which the army has been banished from politics, the economy has boomed and Ankara has pressed its role in the Middle East — portrays the raids and arrests as a “dirty plot” orchestrated by an Islamic cleric. The cleric backs no political party but exercises broad, if covert, influence over the police and judiciary.

Details of the accusations have not been made public but are thought to relate to corruption in construction and real estate projects and in Turkey’s gold trade with Iran, according to Turkish newspaper reports that cite prosecutors’ documents. Prominent business people, the sons of three cabinet ministers and state officials are among those who have been detained for questioning.

The government has hit back by firing or reassigning hundreds of police officers across the country since the crisis broke with a day of raids and arrests on Dec. 17.

About 350 officers in Ankara, the capital, were dismissed or reassigned overnight to new roles, including traffic or district duties, the media reports said. According to the Hurriyet newspaper, about 1,700 officers have been dismissed or reassigned in Istanbul and Ankara since the corruption investigations became public.

Some would have been directly linked to the inquiries, while others may have been removed because of ties to the Hizmet, or Service, movement of the U.S.-based cleric Fethullah Gulen. Erdogan describes it as an intolerable “state within a state.”

Prosecutors, meanwhile, deepened their investigations, with at least 25 more people, including public officials, detained as part of a probe into the activities of a port in the Aegean province of Izmir, broadcaster CNN Turk said.

Eight officials from the state railway company, TCDD, also were detained, the company said.