ISTANBUL (AFP) - Turkish riot police on Saturday (March 5) fired plastic bullets and tear gas to disperse protesters who gathered outside an opposition newspaper the day after it was seized by authorities in a violent raid.

"Free press cannot be silenced," a group of demonstrators shouted outside the Istanbul premises of the Zaman daily, which is staunchly opposed to President Recep Tayyip Erdogan.

Police used large amounts of tear gas, water cannon and plastic bullets to disperse a crowd of around 500 protesters, an AFP photographer at the scene reported.

Before midnight local time on Friday, police had also fired tear gas and water cannon to move away a hundreds-strong crowd that had formed outside the newspaper following a court order placing the media business under administration.

Turkey's top-selling Zaman newspaper, closely linked to Mr Erdogan's arch-foe, the US-based preacher Fethullah Gulen, was ordered into administration by the court on the request of Istanbul prosecutors, local media reported.

The swoop against the paper caused immediate concern in Washington and Brussels over declining media freedoms in Turkey, a key EU ally.

It came ahead of a visit by Prime Minister Ahmet Davutoglu to Brussels on Monday (March 7) for a crucial summit meeting with EU leaders.

Zaman newspaper published a defiant edition on Saturday warning of the "darkest days" in the history of the press.

"The Constitution is suspended," the newspaper said on its front page in large font on a black background.

"Yesterday (Friday) was one of the darkest days in the history of press," it added.

The newspaper, with an estimated circulation of 650,000, went to print earlier than usual on Friday evening, one of its journalists told AFP.

The number of its pages was reduced to 16 from 24 and it was printed before the police raid shortly before midnight local time on Friday.

A team of police arrived with Turkish-made TOMA water cannon trucks and advanced military-style towards the waiting supporters, firing freezing water directly at them.

Using bolt-cutters to unlock the iron gate in front of the building, dozens of police officers then marched into the premises to seize the headquarters and formally place it under administration, pushing aside anyone who stood in their way, media images showed.

Once the building was cleared, the court-appointed administrators - lawyers Tahsin Kaplan and Metin Ilhan and writer Sezai Sengonul - were bussed inside the complex to begin their work, local media reported.

EU enlargement commissioner Johannes Hahn said he was "extremely worried" by the move "which jeopardises progress" made by Turkey in other areas.

He warned on Twitter that Turkey, which is a long-standing candidate to join the European Union, needs to "respect the freedom of the media" and rights were "not negotiable".

The United States said the court order was "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."

"We urge Turkish authorities to ensure their actions uphold the universal democratic values enshrined in their own constitution, including freedom of speech and especially freedom of the press," State Department spokesman John Kirby said.

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

Local media said the court order was issued on the grounds that Zaman supported the activities of this "terror organisation".

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

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

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 Mr Gulen.

The effective seizure of the newspaper by the state added to concerns over freedom of expression in Turkey under Mr Erdogan's rule.

The daily 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.