Can Dundar, editor-in-chief of Turkish daily newspaper Cumhuriyet, and Erdem Gul, the newspaper's Ankara bureau chief, were arrested Thursday night after three hours of questioning at an Istanbul court, according to Turkey's semiofficial Anadolu Agency.
The two journalists have been charged with obtaining and revealing confidential state security information for political or military espionage, and with willfully aiding an armed terrorist organization, the news agency reported.
A Turkish government official, reached by CNN for comment, had little to say.
"This is a judicial process, and we are following it," said the official, who declined to be identified either by name or department.
Group: Turkey 149th in terms of press freedom
Turkey is said by international organizations to have a poor record on press freedom. Reporters Without Borders rates the country 149th of 180 countries
in its 2015 Press Freedom Index.
And in 2013, the Committee to Protect Journalists said more journalists were in prison in Turkey -- 40 -- than in any other country. The number has since dropped to fewer than 10.
"We have been arrested," Dundar tweeted at 11 p.m. Thursday.
Videos of the arrest were shared on Periscope.
In a video before the pair's arrest, Dundar said they were defending freedom of press and "the right of the people to get information."
But Hadi Salihoglu, the Istanbul public prosecutor, disagreed.
"This investigation has nothing to do with 'freedom of press,' which is guaranteed under the constitution and there were no violations of people's rights and freedoms," Salihoglu said, Anadolu Agency reported.
A pro-opposition newspaper
In June, Cumhuriyet published leaked video footage that purported to show the county's intelligence agency sending weapons to Syria.
Cumhuriyet is a secularist and pro-opposition newspaper that has been in circulation since 1924.
The video showed men in police uniforms and civilian clothing unscrewing bolts to open the cargo area of the trucks, and unpacking boxes of what look like medicine. Later images show trucks full of mortar rounds.
Cumhuriyet reported that the weapons were being delivered to Syria by Turkey's National Intelligence Agency.
"The weapons were most likely headed to ISIS, infamous globally for its brutality," Dundar told CNN.
CNN cannot independently confirm the authenticity of the video.
Turkish President Recep Tayyip Erdogan responded swiftly to the leak.
"The person who published this story as an exclusive story will, I believe, pay a high price for this," Erdogan said on state broadcaster TRT. "I will not leave him be."
An investigation of Dundar and Gul was launched for "treason and revealing state secrets," and it led to the men's arrest Thursday.
The video footage was removed from websites. Sites that did not comply with a court order to remove it were blocked on the basis of national interest and national security.
Erdogan: It was aid for Syrian Turkmens, 'our kin'
The Turkish President and Prime Minister have repeatedly said the aid was going to Turkmen groups in Syria.
Those Turkmen groups have been a central aspect of the recent conflict between Turkey and Russia.
Turkey says Russia is targeting moderate Syrian opposition groups, including Turkmens, in its aerial bombardment in Syria. Russia, though, has said it is targeting ISIS.
Erdogan told CNN that Turkey would continue providing humanitarian aid to the Turkmens, who are, he said, "our kin."
'Journalists cannot be spies'
Tayfun Atay, a colleague at Cumhuriyet and longtime friend of Dundar's, told CNN that the current political climate punishes opposition journalists.
Dundar and Gul "did not give in," Atay said.
"Journalists cannot be spies because journalists do not hide but reveal information for the good of the public," he said. "I am proud of them."
Mehmet Yilmaz, columnist at daily Hurriyet newspaper, was also critical.
"The arrests show that there is no freedom of expression in Turkey," Yilmaz said.
Reporters Without Borders recognized Cumhuriyet in its Reporters Without Borders Prize for Press Freedom on November 17.
The journalists face life in prison if convicted.