ANKARA/ISTANBUL (Reuters) - At the FEM University Preparation School in Uskudar, a conservative district on the Asian side of Istanbul, young men are quietly receiving specialised coaching in how to pass the exams that give access to the most important jobs in Turkey.
To a casual eye, nothing seems remarkable. As in nearly all Turkish schools, a portrait of modern Turkey's secular founder Mustafa Kemal Ataturk hangs in every classroom. Ataturk's address to youth hangs on the wall at the school's entrance.
There are no visible references either to the religious movement which runs the school, known as Hizmet, or "Service" - or to the movement's founder, cleric Fethullah Gulen, based in the United States for 14 years. But the teachers are almost all Gulen followers, as are many of the pupils and their parents.
The government wants to shut schools like this one down, officially arguing that test preparation academies provide an unfair advantage and place a financial burden on families who feel they must pay tuition or their children will under-perform.
But for Gulen supporters, the proposal is just the latest attempt by Prime Minister Tayyip Erdogan to curb a movement that has emerged as a challenge to his domination of the country.
Gulen's followers are believed to control as many as a quarter of the exam preparation academies in Turkey, giving a seemingly innocuous religious movement an outsized role in shaping the views of the country's future elite.
"My family directed me here. They wouldn't want me to go to another prep school, nor would I. They like this movement and want me to be involved too," said Taha Ramazan Sisman, 18, who says he plans to study medicine when he enters university this year - and then serve the movement upon graduation.
Just what Hizmet actually is depends on whom you ask. Supporters say members are free to join and leave, and take no orders from above. They define themselves mainly as conservative Muslims who believe in the importance of education and charity.
To Erdogan, an Islamist who once drew on Hizmet for support, the organisation has grown into the biggest threat to his own 11-year domination of the country, a "state within a state" that must be dismantled.
Since a corruption probe late last year pitted police and judges - many of them Gulen followers - against senior politicians and business associates of the prime minister accused of graft, Erdogan and Gulen have become arch enemies.
The prime minister has denounced the corruption investigations as a foreign plot. The authorities have purged hundreds of police, many of them Gulen followers.
Gulen has lashed out against the government, praying "God bring fire to their houses".
Erdogan, who does not mention Gulen by name but makes his target clear, has vowed to root out the nation's foes in their "lairs" and accused them of laying "wicked and dark traps in our country".
The corruption probe and dispute with Gulen supporters have finally dented the seemingly indestructible Erdogan, who had emerged largely unscathed from mass anti-government street demonstrations last year.
"The government was seriously damaged," Ozer Sancar, head of MetroPoll research said. "This is a far bigger challenge than the summer protests as it involves corruption. How Erdogan will handle the crisis will determine the electorate's behaviour."
If Gulen now poses the greatest threat to Erdogan, it is at least in part a challenge of the prime minister's own making.
Erdogan's Islamist AK Party long encouraged Gulen's followers to seek positions in the bureaucracy to displace the "deep state" - the army-backed secularist establishment that had ruled Turkey since the fall of the Ottoman Empire.
"When Erdogan came to power in 2002, he didn't have the personnel to occupy key positions," an official close to Erdogan's office said. "His stream of the Islamist tradition had not really invested in democratic institutions in the past, but Hizmet had spent decades training professionals in its schools. So he called on them."
Hizmet helped cement the rise of Erdogan's AK Party. Its influence in the judiciary - which jailed senior officers on coup plot charges - was widely seen as crucial to taming an army which toppled three governments in the latter 20th century and forced an Islamist-led cabinet from power in 1997.
However the marriage of convenience appears to have soured in the past year or so, with the discord blowing thunderously into the open with the corruption investigation last year.
Erdogan's backers say Hizmet's appetite for influence had become relentless, to the point that members were demanding control of whole state departments.
"The movement already has thousands of people in the state, but they always ask for more," said a government official, asking not to be identified while discussing Hizmet's influence behind the scenes.
"You hire one person from the movement in a certain department. Then two more, then five more. And the next day they complain: 'Why isn't the whole department made up of Hizmet people?'"
Senior officials have said a criminal investigation will soon be launched into the formation of "an illegal organisation within the state", a charge Hizmet followers say they will defend themselves against.
"If there is concrete and conclusive evidence ... this evidence should be referred to the judicial authorities for investigation," the Journalists and Writers Foundation, a group which has Gulen as its honorary leader and which has spoken on behalf of his movement in the past, said on Tuesday.
"It is a hate crime to repeat this slander again and again without producing any evidence or taking legal action."
Born in Erzurum, eastern Turkey, 72 years ago, Gulen built up his reputation as a Muslim preacher with intense sermons that often moved him to tears. From his base in Izmir, he toured Turkey stressing the need to embrace scientific progress, shun radicalism and build bridges to the West and other faiths.
The first Gulen school opened in 1982. In the following decades, the movement became a spectacular success, setting up hundreds of schools that turned out generations of capable graduates, who gravitated to influential jobs in the judiciary, police, media, state bureaucracy and private business.
Its millions of followers refer to Gulen as "Hocafendi", or "respected teacher". They have branched out from Turkey to other countries, particularly the Turkic language-speaking former Soviet republics of Central Asia, and African countries, where they fund schools and open businesses.
Like the Mormons of the United States or the Ismaili Muslims of South Asia and elsewhere, members are expected to contribute a share of their income to the movement's charitable work, a source of funding that has helped build the schools and colleges that produce the graduates that expand its reach.
His followers say Gulen aims to educate a "golden generation" of conservative, well-travelled and patriotic young Muslims who will promote Turkish values around the world.
His emphasis on education, free market economics and integration into Europe drew middle class followers, who in turn earned influence through their own successful careers and built more schools with their donations.
"The main principle of the movement is not to have any expectations and to always give more than you take. The supporters usually donate a certain amount of their revenues to social responsibility projects," said Rizanur Meral, head of TUSKON, Hizmet's association of businessmen, which boasts 54,000 individual members who own 140,000 companies.
"There is a really important principle that we learnt from Hocafendi: if we receive a worldly benefit from our charitable work, then we will not get our reward in the afterlife. So supporters are never after material rewards," he explained.
In 1999, Gulen decamped to a compound in the Pocono mountains of rural Pennsylvania, shortly before the start of a case against him in Turkey on charges of plotting to destroy the secular state and establish Islamic law. He was acquitted but has remained in the United States.
Opponents describe Hizmet almost as a conspiracy.
Former police chief Hanefi Avci, a conservative Muslim whose children graduated from Hizmet schools but who landed in jail after writing a book lifting the lid on the group's workings within the force, describes a systematic network.
"In the intelligence and smuggling and organised crime units of police command, a majority of the technical and managerial teams are (Hizmet) members or are carrying out instructions coming from the group," wrote Avci.
"This organisation is not confined to the police command. The group has a structure pretty much in every institution like the MIT (intelligence service), the army, judiciary, even members of parliament," said Avci, who was detained weeks after the book was published in 2010 and remains in jail, convicted of links to a leftist terrorist organisation.
Communication among members, one official close to the government said, is discreet and conducted largely by word of mouth: "You start to hear the same demands or view arising from different quarters, often couched in the same language. You understand where it's coming from."
Another senior Turkish official described the method as building influence "without leaving fingerprints".
Such an intangible structure means curbing Hizmet's power could prove even more challenging for Erdogan's administration than taming the military, say some Turkey watchers.
"They're more careful than the military about revealing who is among them. They worked hard to put people in government," said James Jeffrey, a former U.S. ambassador to Turkey, now a fellow at the Washington Institute for Near East Policy think tank. "They're not going away."
(Additional reporting by Nick Tattersall and Ayla Jean Yackley; Writing and editing by Ralph Boulton and Peter Graff)