Opposition groups have criticised the Ergenekon investigation for stifling free press and democracy [EPA]

A Turkish court has charged four journalists and a writer with involvement in an alleged plot to overthrow the country's government, a day after two prominent reporters were jailed pending trial.

Yalcin Kucuk, a staunch critic of the ruling administration, was charged and jailed on Monday along with four journalists from the anti-government Odatv website. All five were charged with membership of a terrorist group and stirring hatred.

On Sunday Nedim Sener and Ahmet Sik, two leading investigative journalists who have written books about secret activities within the state, were also charged and jailed on similar allegations.

The charges came a week after the group of seven were detained following police raids on their homes.

Turkey's government is conducting a lengthy investigation into the ultra-nationalist Ergenekon group, which is accused of plotting to overthrow the adminstration of Recep Tayyip Erdogan, the prime minister, by planning attacks in order to provoke a military coup.

But human rights groups and the European Union have voiced concern over the crackdown, which has seen hundreds of opposition figures jailed since 2007, saying it raises questions over Turkey's commitment to democracy and freedom of speech.

Human Rights Watch said on Saturday the developments in the case had a "chilling effect" on free speech and urged Erdogan's government to respect press freedom. Thousands of people also marched in Istanbul and Ankara, the capital, on Friday, to protest against the detentions.

Besir Atalay, the interior minister, has rejected allegations of stifling free press saying: "It is an injustice that the government should be exposed to allegations of silencing the voice of the press or limiting press freedom."

Zekeriya Oz, the prosecutor at the heart of the investigation into Ergenekon, also said the charges brought on Sunday and Monday were not related to journalistic activities.

"This is a legal process which became necessary after assessing evidence ... and is not related to the journalistic duties, writings, books and views of a group of press members."