* 10 journalists and writers detained
* Accused of membership of coup-plotting group
* State Department concerned about trends in Turkey
* Journalists hold protests in Istanbul, Ankara
(Adds Erdogan, writer convicted)
By Daren Butler
ISTANBUL, March 4 (Reuters) - Several thousand people protested in Turkey 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 part of an investigation into an alleged conspiracy to bring down the government. Critics of the government say the investigation 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 organised by journalists through central Istanbul on Friday, demanding the release of their colleagues. Some 60 journalists are in jail.
"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.
Erdogan defended himself against opposition accusations of political meddling in the judiciary on Friday, calling for the media and opposition to "behave responsibly" over the case.
"We are not prosecutors, judges or lawyers. We, as the executive, only help the judiciary with the security forces," the prime minister said in a speech on Friday.
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.
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.
A court on Friday sentenced academic Ismail Besikci to 15 months in prison for a magazine article he wrote about the Kurds which was deemed "terrorist propaganda". Besikci has previously spent 17 years in prison on similar charges.
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."
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. (Editing by Simon Cameron-Moore and Jon Hemming)