Police use tear-inducing agent against demonstrators during a protest over the arrest of journalists Can Dundar and Erdem Gul in Ankara on Friday.

AFP/Istanbul

Hundreds of people demonstrated in Turkey on Friday in support of two journalists from a leading newspaper being held on spying charges over a report suggesting Ankara shipped arms to rebels in Syria.

Over 1,000 demonstrators, including a number of journalists and opposition MPs, gathered outside the Istanbul offices of Cumhuriyet daily shouting slogans such as "Shoulder to shoulder against fascism".

In Ankara, police fired tear gas to disperse a similar protest by a crowd of around 700, an AFP photographer said.

The demonstrations came a day after an Istanbul court charged Cumhuriyet's editor-in-chief Can Dundar and Ankara bureau chief Erdem Gul with "aiding a terrorist organisation" and spying for alleging that Turkey, a fierce critic of Syrian President Bashar al-Assad, had covertly shipped arms to Syria.

The pair are also accused of leaking state secrets "for the purposes of military or political espionage". Taken together the charges carry up to 45 years in prison, the paper confirmed to AFP.

Cumhuriyet reported that Turkish security forces in January 2014 intercepted a convoy of trucks near the Syrian border carrying boxes of what it described as weapons and ammunition bound for Syria, ostensibly for rebels fighting the Assad regime.

The footage, which was published on the newspaper's website in May, showed police opening crates of weapons and ammunition on the back of trucks which the daily claimed belonged to the Turkish Intelligence Organisation (MIT).

The report sparked a furore in Turkey, fuelling speculation about the government's role in the Syrian conflict and its alleged relations with the Islamic State jihadists leading the fight against Assad's forces in some areas.

Ankara has fiercely rejected allegations of complicity with IS, which it blames for two major suicide attacks this year in Turkey.

President Tayyip Erdogan had warned Dundar would "pay a high price" for the report.

'Chilling message'

Reporters Without Borders ranked Turkey 149th out of 180 in its 2015 press freedom index last month, warning of a "dangerous surge in censorship".

The US government, speaking through its embassy in Ankara, and the European Union voiced concerns over the arrests.

"We are very concerned by the arrests .... and what appears to be yet another media outlet under pressure," the US embassy wrote on Twitter on Friday.

"We hope the Turkish courts & authorities will uphold the fundamental principle of media freedom enshrined in the Turkish Constitution."

The European Union described the arrests as "worrying".

"We are of course following these worrying developments very closely," European Commission foreign affairs spokeswoman Maja Kocijancic told a daily briefing, two days before EU and Turkish leaders meet in Brussels.

"Freedom of expression is one of the fundamental principles for the EU (and) ... we have raised these concerns repeatedly with the Turkish authorities," Kocijancic said.

The Organisation for Security and Co-operation in Europe (OSCE) said the charges represented yet another blow for press freedom in Turkey, where media critical of Erdogan have been targeted in a crackdown.

"Journalists facing life in prison for their reporting deemed dangerous by the authorities is unacceptable", the OSCE's media freedom representative Dunja Mijatovic said on Friday.

"Even the prospect of such harsh punishment sends a chilling message to society that disagreeing with the views of those in power will be severely punished."