Zaman media group employees and their relatives hold banners outside the headquarters of Zaman daily newspaper in Istanbul December 14, 2014, after Turkish police raided media outlets. (Murray Sezer/Reuters)

Turkish authorities seized control of the country's largest newspaper on Friday, state-run media said, in a widening crackdown against supporters of US-based Muslim cleric Fethullah Gulen, an influential foe of President Tayyip Erdogan.

Administrators have been appointed to run the Feza Media Group, which owns the daily Zaman newspaper and several other papers, at the request of an Istanbul prosecutor, state-run Anadolu Agency reported.

An article on the paper's English-language edition's website, Today's Zaman, said the court's decision "means that the entire management and the editorial board of Feza Media Group companies will be replaced by the three-member board named by the court."

The paper said the board would be made up of an editor and two lawyers, one of whom, Metin Ilhan, is an open supporter of Erdogan and his AK Party.

Officials were not immediately available to confirm the reports.

Watch footage of police confronting protesters at the newspaper's offices:

The move against Zaman came hours after police detained prominent businessmen over allegations of financing what prosecutors described as a "Gulenist terror group," Anadolu reported.

Erdogan accuses Gulen of conspiring to overthrow the government by building a network of supporters in the judiciary, police and media. Gulen denies the charges. The two men were allies until police and prosecutors seen as sympathetic to Gulen opened a corruption probe into Erdogan's inner circle in 2013.

"It has been a habit for the last three, four years, that anyone who is speaking against government policies is facing either court cases or prison, or such control by the government," said Abdulhamit Bilici, editor-in-chief of Zaman.

"This is a dark period for our country, our democracy."

Zaman is Turkey's biggest selling newspaper, with a circulation of 650,000 as of the end of February, according to media-sector monitor MedyaTava website.

Hundreds of supporters gathered in the rain outside Zaman's Istanbul office where they waved Turkish flags and carried placards reading "Hands off my newspaper" and "Free media cannot be silenced," live footage from Cihan, a broadcaster owned by the same parent as Zaman, showed.

The crackdown on Zaman comes at an already worrying time for press freedom in Turkey.

Two prominent journalists from the pro-opposition Cumhuriyet newspaper are facing potential life sentences on charges of endangering state security for publishing material that purports to show intelligence officials trucking arms to Syria.

Authorities have seized and shut down opposition media outlets associated with the Gulen movement before. The state deposit insurance fund said this week an Islamic bank founded by Gulen followers might be liquidated within months.

Gulen's movement has adherents in the United States, Africa, and Asia, where it runs private schools and says it promotes interfaith dialogue.

Earlier on Friday police detained Memduh Boydak, chief executive of furniture-to-cables conglomerate Boydak Holding, as well as the group's chairman Haci Boydak and two board members, Anadolu reported.

Haci Boydak and Memduh Boydak were accused of being members of the "Gulenist terror group" and providing it with financial support, Anadolu said. Board members Murat Bozdag and Erol Boydak were accused of spreading the group's propaganda on social media, it said, adding that all were detained at home.

Nobody from the company, based in the central Turkish city of Kayseri, was available to comment

Erdogan has accused Gulen of operating a "parallel state structure" bent on toppling him.

Government officials have also accused Gulen's followers of having ties to the outlawed Kurdistan Workers Party (PKK). Gulen denies such links and describes the PKK as a terrorist group.

The US State Department on Friday called the decision by Turkish authorities to seize the newspaper "troubling."

"We see this as the latest in a series of troubling judicial and law enforcement actions taken by the Turkish government targeting media outlets and others critical of it," State Department spokesman John Kirby told a news briefing.

Amnesty International, Reporters without Borders, and other press freedom and human rights organizations were quick to condemn the move, as well. Freedom House Executive Vice President Daniel Calingaert said the Turkish government's seizure of _Zaman _"amounts to a government takeover of a private media outlet, and is a flagrant violation of both rule of law and freedom of the press."