Istanbul (AFP) - A Turkish court on Friday freed four academics from jail on the first day of their trial for spreading "terrorist propaganda", as prosecutors moved to scale back the charges against them.

The four, on trial for signing a petition denouncing the government's military operations against Kurdish rebels, were released "pending permission from the justice ministry" to change the charge, lawyer Benan Molu told AFP.

Under the original charge, Esra Mungan Gursoy, Meral Camci, Kivanc Ersoy and Muzaffer Kaya faced up to seven-and-a-half years behind bars.

But prosecutors now want to slap them with charges under Article 301, a law which states that "denigrating Turkishness" is a criminal act -- which carries a maximum sentence of two years in jail.

Supporters in court applauded as they walked free, with the judge setting setting the next hearing for September 27.

Earlier, Kaya had fiercly defended the petition, telling the court that the state had "not managed to stifle the voices of our conscience" and that they had been arrested for "criticising political power", Dogan news agency said.

"You may find our petition ridiculous, but you can never say we were spreading terrorist propaganda. Acquit me," he said.

Riot police had stood guard outside the courthouse in central Istanbul, where the academics' trial had followed a morning hearing in the case of two journalists accused of divulging state secrets.

- 'A pit of treachery' -

Some 500 people had gathered at the court to support both the journalists and the scholars, with protesters holding up placards reading "Freedom for the academics" and "Freedom for the pencils".

The petition had urged Ankara to halt "its deliberate massacres and deportation of Kurdish and other peoples in the region", infuriating Erdogan who accused them of falling into a "pit of treachery".

The four stood accused of engaging in "terrorist propaganda" and "inciting hatred and enmity" for not only signing the plea but making a statement on the same lines on March 10, a day before the petition was published.

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They had been held in high-security closed prisons in Istanbul since their arrest last month.

As well as signees from over 90 Turkish universities, the petition was also endorsed by dozens of foreigners, among them American linguist Noam Chomsky and the Slovenian philosopher Slavoj Zizek.

Turkey is waging an all-out offensive against the separatist Kurdistan Workers' Party (PKK), with military operations backed by curfews aimed at flushing out rebels from several southeastern urban centres.

But Kurdish activists say dozens of civilians have died as a result of excessive force.

- 'A step toward acquittal' -

The decision to haul scholars and journalists into court has deepened unease over freedom of expression under the increasingly-autocratic Erdogan.

The US and European Union have already expressed concern over the trial of Dundar and his Ankara bureau chief Erdem Gul who face life behind bars over a story accusing the government of seeking to illicitly deliver arms to Islamists in Syria.

Friday's hearing was dedicated to the prosecution's request to merge the journalists' trial with another in which former politicians and intelligence officials are accused of trying to overthrow the government -- a request the court denied.

"Their plan was smashed to bits. We are journalists and have nothing to do with that case, the court confirmed that.

"I think we made a step towards acquittal," Dundar said as the hearing ended. The next hearing was set for May 6.

In the 2016 World Press Freedom Index, Turkey slipped two places to 151 out of 180, Reporters Without Borders said Wednesday, citing the president's "offensive" against the media and his critics.

Almost 2,000 people have been prosecuted for "insulting" Erdogan since the former premier became president in August 2014, Turkey's justice minister said in March.