Factbox: Turkish government's battle with U.S.-based cleric Gulen

ISTANBUL (Reuters) - Turkish police have raided media outlets close to U.S.-based Muslim cleric Fethullah Gulen, in what President Tayyip Erdogan defended as a necessary response to "dirty operations" by the government's political enemies. Erdogan accuses Gulen of establishing a 'parallel' structure in the state through his supporters in the judiciary, police and other institutions, and of wielding influence through the media, part of an orchestrated attempt to usurp him. The cleric, a former ally of Erdogan whose fall-out with him became public after corruption charges were brought against the government last year, denies any ambition to overthrow him. The weekend raids were not the first time the government has tried to "cleanse" state institutions of what it says is Gulen's influence. Following are the details of other purges: POLICE Thousands of police officers were reassigned or removed from their posts in the months after anti-graft raids in December 2013 that touched on Erdogan's inner circle, and which he blamed on "treasonous" elements within the force. Serving and dismissed officers interviewed by Reuters said the purge hit the financial, organized crime, smuggling and anti-terrorism units. Thousands were dismissed or reassigned to other tasks, including traffic duty, they said. The national police declined to comment on the scale of the purge or whether it undermined the force. Dozens of police including high-ranking officers were also detained in July, accused of spying and illegal wire-tapping of Erdogan and his inner circle. JUDICIARY Erdogan has likened Gulen's followers in the judiciary to medieval "Assassins" betraying Turkey for what he sees as their role in the corruption investigations. Hundreds of judges and prosecutors have been reassigned. Government-backed candidates won most seats in an October vote for the High Board of Judges and Prosecutors council (HSYK), responsible for appointments, transfers, promotions and expulsions in the judiciary. The government also pushed through legislation this month increasing police search powers and reforming the top two courts, the Court of Appeals and the Council of State. Justice Minister Bekir Bozdag said the reform would ease judges' workload but one senior judge said it threatened to erode the judicial culture of the country. EDUCATION Parliament voted in March to close private preparatory schools, many of which are a source of income and influence for Gulen's "Hizmet" (service) movement. Hizmet says education is central to its mission, with millions of students preparing at the centers for entrance examinations to win limited spots at state high schools and universities. The system also helped Gulen spread influence across a nationwide network. Hizmet members have said there is also pressure on governments abroad to shut down Gulen schools as far away as Africa and Asia, as well as to discourage Hizmet-linked commerce from banking to construction. The government accuses Hizmet overseas of running a propaganda campaign against the Turkish state. BUSINESS Islamic lender Bank Asya , founded by Gulen sympathizers, has seen depositors including state-owned firms and institutions withdraw funds this year in what it has described as a systematic campaign to undermine it. Pro-government newspapers carried almost daily reports on Bank Asya's woes earlier this year, portraying it as a failing bank being propped up by members of Gulen's network. The government canceled tax collection and social security payment contracts with Bank Asya in August. Erdogan has denied any concerted effort to sink the bank, which has said it is meeting all its obligations. MEDIA Before Sunday's raids, Turkish media close to Gulen had been shut out of government press events. Correspondents from the Zaman and Bugun newspapers, the Samanyolu TV station and Cihan news agency said they had been banned from the presidential palace since Erdogan's inauguration as president in August and no longer received press releases. Erdogan has repeatedly described Gulen's network as a threat to national security. The media ban was extended to key ministries including the prime minister's office and the ministry of foreign affairs a few days after a National Security Council meeting in October, according to journalists from Gulen-linked media outlets. (Writing by Nick Tattersall; Editing by Asli Kandemir)