Istanbul (AFP) - An Istanbul court on Friday ordered into administration a Turkish daily newspaper that is sharply critical of President Recep Tayyip Erdogan, adding to growing alarm over freedom of expression in the country.

The Zaman newspaper, closely linked to Erdogan's arch-foe the US-based preacher Fethullah Gulen, was ordered into administration by the court on the request of Istanbul prosecutors, the state-run Anatolia news agency said.

There was no immediate official explanation for the court's decision.

The move means the court will appoint new managers to run the newspaper, who will be expected to transform its editorial line.

Hundreds of supporters were gathered outside the paper's headquarters in Istanbul awaiting the arrival of bailiffs and security forces after the court order.

"We will fight for a free press," and "We will not remain silent" said placards held by protestors, according to live images broadcast on the pro-Gulen Samanyolu TV.

"Democracy will continue and free media will not be silent," Zaman's editor-in-chief Abdulhamit Bilici was quoted as saying by the Cihan news agency outside its headquarters.

"I believe that free media will continue even if we have to write on the walls. I don't think it is possible to silence media in the digital age," he told Cihan, part of the Zaman media group.

- 'Last free edition' -

It was not yet clear when the bailiffs would arrive. Sevgi Akarcesme, the editor-in-chief of the paper's English language edition Today's Zaman, said on Twitter that staff were working on the "last free edition" of their newspaper.

Gulen has been based in the United States since 1999 when he fled charges against him laid by the former secular authorities.

Turkey has asked the United States to extradite him but Washington has shown little appetite for doing so.

Despite living outside of Turkey, Gulen built up huge influence in the country through allies in the police and judiciary, media and financial interests and a vast network of cramming school.

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Ankara now accuses Gulen of running what it calls the Fethullahaci Terror Organisation/Parallel State Structure (FeTO/PDY) and seeking to overthrow the legitimate Turkish authorities.

There have been numerous legal crackdowns on structures linked to the group and on Friday Turkish police arrested four executives of one of the country's largest conglomerates, accusing them of financing Gulen.

Boydak Holding group president Haci Boydak, director general Memduh Boydak and two board members were questioned at their homes in the central city of Kayseri.

Gulen supporters decry the accusations as ridiculous, saying all he leads is a more informal group known as Hizmet (Service).

- 'Just before EU summit' -

The effective seizure of the newspaper by the state comes amid growing concerns over freedom of expression in Turkey under Erdogan's rule.

The Cumhuriyet newspaper's editor-in-chief Can Dundar and Ankara bureau chief Erdem Gul were released on an order from Turkey's top court last week after three months in jail on charges of publishing state secrets.

But they still face trial on March 25.

Meanwhile almost 2,000 journalists, bloggers and ordinary citizens, including high school students, have found themselves prosecuted on accusations of insulting Erdogan.

"By lashing out and seeking to rein in critical voices, President Erdogan's government is steamrolling over human rights," said Andrew Gardner, Amnesty International's Turkey expert in a statement.

Independent pro-Kurdish television channel IMV TV was taken off air in Turkey last weekend following accusations that it broadcast "terrorist propaganda" for militants.

The decision on Zaman comes as Prime Minister Ahmet Davutoglu heads to Brussels Monday for a crucial summit meeting with EU leaders. The news broke as Erdogan was holding talks in Istanbul with EU President Donald Tusk.

Critics have accused the EU of turning a blind eye to the situation with media freedom in Turkey in exchange for Ankara's cooperation in the refugee crisis.

Opponents say Erdogan, who served as premier from 2003-2014, has become an increasingly polarising figure unable to tolerate any criticism.

However the government angrily dismisses allegations it is cracking down on the press, saying the cases against Cumhuriyet and pro-Gulen media have nothing to do with freedom of expression.