Journalists in Turkey marched in protest on Friday after police arrested 10 reporters and writers, detentions that prompted the European Commission to warn the EU candidate country over its democratic credentials.
Thursday's arrests were ordered as part of a widening conspiracy investigation that government critics say is being used to hound them.
Two weeks ago three other journalists were jailed pending trial on charges of ties to a murky ultra-nationalist group known as Ergenekon, alleged to have plotted to overthrow the ruling AK Party government.
"The European Commission is following with concern the recent police actions against journalists," EU Enlargement Commissioner Stefan Fuele said in a statement late on Thursday.
Several thousand people joined a protest march organized by journalists through central Istanbul on Friday, demanding the release of their colleagues. One placard indicated there were 61 journalists who should be freed.
"The free press cannot be silenced," marchers chanted before switching to, "Damn the AKP dictatorship".
The protest in the capital Ankara, was smaller, with about 500 mostly journalists protesting at a lack of media freedom under Prime Minister Tayyip Erdogan's government. Some bound their mouths with black tape as they bore aloft placards.
In its progress report on EU-membership candidate Turkey, the European Commission had highlighted the high number of court cases against journalists. Many reporters are being investigated over their coverage of alleged plots to topple the government.
Hundreds of people, from military officers to academics and politicians are being tried in those cases, which reflect deep mistrust between the secular establishment and Erdogan's AK.
Critics say the party has Islamist leanings and see the Ergenekon investigations as targeting opponents of the government. But Erdogan told reporters on Thursday evening the detentions had nothing to do with the government.
Fuele said freedom of expression and media were fundamental principles which should be upheld in all modern democracies.
"As a candidate country, we expect Turkey to implement such core democratic principles and enable varied, pluralistic debate in public space," he said.
"Turkey urgently needs to amend its legal framework to improve the exercise of freedom of the press in practice and in a significant manner," he added.
State Department spokesman P.J. Crowley told a news briefing the United States was monitoring the case, urging a transparent investigation and an independent, pluralistic media.
"We have concerns about trends in Turkey, as we have indicated publicly. We continue to engage Turkish officials on these developments and we will follow these cases very closely."
The Ergenekon investigation had fuelled hopes at its outset of an end to the era of military coups, but Turkey was still waiting for an outcome after years of countless detentions, TUSIAD business association head Umit Boyner said.
Turkey's military staged three coups between 1960 and 1980 and toppled another government in 1997. However its power has since been dramatically curbed under EU-inspired reforms and further military intervention is seen as highly unlikely.
"How much longer will we wait for democracy, transparency and justice to be achieved? What is the expiry date for the question of what this is all about?" Boyner said in a written statement to the state-run Anatolian news agency.
One of those arrested on Thursday, Ahmet Sik, is already on trial over a book he co-wrote about the Ergenekon investigation.
Sik was writing a book on links between the police and the Gulen Islamist movement, a draft of which was found by police in a raid on a website's offices two weeks ago, media reports say.