The US and EU have criticised Turkey over the police crackdowns and more than two dozen detentions, targeting two opposition media outlets.
Istanbul police raided on Sunday the country’s most-read newspaper and a national television close to Fetullah Gulen, an influential US-based religious leader and a former government ally, and detained 27 people, including top executives of the outlets in question.
Various materials, including hard drives, were seized following the police search at Zaman newspaper and Samanyolu TV buildings. A police chief is also among the ones detained.
Semi-official Anatolia news agency reported on Monday that four detainees, who worked for Samanyolu TV, were released after questioning.
In a press briefing on Sunday, the US State Department spokesperson called on Turkish authorities to protect media freedom and other democratic values.
"Media freedom, due process, and judicial independence are key elements in every healthy democracy and are enshrined in the Turkish constitution," Jen Psaki said.
"As Turkey's friend and ally, we urge the Turkish authorities to ensure their actions do not violate these core values and Turkey's own democratic foundations."
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"The police raids and arrests of a number of journalists and media representatives in Turkey today are incompatible with the freedom of media, which is a core principle of democracy," Federica Mogherini, EU foreign policy chief, and EU Enlargement Commissioner Johannes Hahn said in the joint statement.
"This operation goes against the European values and standards Turkey aspires to be part of."
Hadi Salihoglu, Istanbul chief prosecutor, said in a statement on Sunday arrest warrants had been issued for 31 people on charges of "establishing a terrorist group", forgery and slander.
"We are facing a real monster that has gone haywire, focused on toppling every democratic stronghold, determined to annihilate its every target with no regard for law and order through violence rage and hate," Ekrem Dumanli, editor-in-chief of Zaman, said just before he was detained.
"This is a shameful sight for Turkey," Hidayet Karaca, Samanyolu TV chairman, said.
"Sadly in the 21st century Turkey, this is the treatment they dish out to a media group with tens of television and radio stations, Internet media and magazines."
Last December, Turkey was shaken by the most extensive and sensational corruption investigations of its recent history that have led to dozens of detentions - from renowned business people to senior bureaucrats and sons of ministers.
Recep Tayyip Erdogan, Turkey's president, accuses the Gulen Movement of establishing a "parallel structure" in state institutions through his supporters in the judiciary, police and other institutions, while enjoying public influence through his affiliated media outlets.
Erdogan and officials from ruling Justice and Development (AK) Party have at various times claimed that last December’s corruption investigations were a plot by the Gulenists to topple the government.
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After the investigations were launched, the government carried out a country-wide comprehensive overhaul in the Turkish police force, and various officers were arrested and charged.
In February, the government also passed a new bill to give the justice ministry greater sway over the Supreme Board of Judges and Prosecutors, or HSYK, an independent body responsible for appointing members of the judiciary.
Turkish government wields high influence over the Turkish media.
In phone recordings revealed earlier in 2014, Erdogan, who was prime minister at the time, is heard giving directions to a board member of a national media network about their television and newspaper new coverage.
And several renowned journalists allegedly lost their jobs after Erdogan publicly condemned them.
Source: Al Jazeera and agencies