Several thousand people protested on Friday over the arrest of two prominent journalists on charges of espionage and terrorist propaganda, a case that has revived long-standing criticism of Turkey's record on press freedom under President Tayyip Erdogan.

A court on Thursday ordered the arrest of Can Dundar, editor-in-chief of Cumhuriyet newspaper, and senior editor Erdem Gul over the publication of footage purporting to show the state intelligence agency helping send weapons to Syria.

The United States said it was "very concerned," and opposition politicians fiercely criticised the move.

"Journalism is being put on trial with these arrests and the Turkish press is being intimidated," Utku Cakirozer, a deputy from the main opposition People's Republican Party (CHP) and Cumhuriyet's former top editor, told Reuters.

Some 2,000 people gathered in Istanbul, with some chanting "Murderer Erdogan" and accusing the ruling AK Party he founded of collaborating with ISIS. Some demonstrators held up Friday's edition of Cumhuriyet, which carried the headline "Black day for the press". Cumhuriyet is a secular, left-wing newspaper that is often critical of the government.

"All opposition press organisations that are abiding by the ethics of journalism and trying to do their journalism are under threat and under attack," Figen Yuksekdag, co-chairwoman of the pro-Kurdish Peoples' Democratic Party, said at the protest.

"This dark operation aimed at covering the crimes that those trucks carried and the crimes which are continuing to be committed will not be successful," she added.