About 300 people on Sunday protested the arrest of two prominent journalists on charges of spying over their reports about Ankara's alleged arms supplies to Syrian rebels in a case that has revived long-standing criticism of Turkey's press freedom record.
The demonstrators, including journalists, rights groups and opposition MPs, marched to the Istanbul offices of the Cumhuriyet daily to express solidarity with its editor-in-chief Can Dundar and Ankara bureau chief Erdem Gul.
"Shoulder to shoulder against fascism," they shouted, according to AFP.
They held placards that read: "We stand up for press freedom. Free press cannot be silenced."
Addressing the crowd outside Cumhuriyet, the main opposition Republican People's Party's (CHP) leader Kemal Kilicdaroglu denounced the arrest as a "black stain on the history of press."
"We have a common responsibility to protect our media and our democracy," he said.
"Dundar will get out and reveal the secrets again," the crowd shouted back.
A court in Istanbul on Thursday arrested Dundar and Gul for "divulging state secrets" over the publication of footage from January 2014 purporting to show Turkey's secret services helping send weapons to Islamist rebels in Syria.
They face up to 45 years in jail if convicted, according to AFP.
The revelations, published in May, caused a political storm in Turkey, with an enraged President Recep Tayyip Erdogan vowing Dundar would pay a "heavy price".
There has been growing concern about deteriorating press freedoms in Turkey under Erdogan and in particular over the numbers of journalists facing legal proceedings on accusations of insulting or criticizing top officials.
In June, a reporter from Cumhuriyet was one of four journalists who were detained by Turkish forces after they asked the local governor a tricky question on Islamic State (ISIS) terrorists.
Last year, Turkish police earlier raided media outlets close to U.S.-based Muslim cleric Fethullah Gulen, whom Erdogan accuses of forming a "parallel state" to undermine his rule and orchestrating a graft scandal targeting his inner circle.
When the European Union (EU) criticized Erdogan over the raids, he responded by saying EU leaders should try to find a solution for the increasing Islamophobia in Europe instead of criticizing Turkey.
AFP contributed to this report.