Prominent journalists and other staff at a Turkish opposition newspaper went on trial on Monday accused of supporting a terrorist group, in a case that critics of President Tayyip Erdogan consider attack on free speech.

"Journalism is not a crime," chanted several hundred people gathered outside the central Istanbul court to protest against the prosecution of 17 writers, executives and lawyers of the secularist Cumhuriyet newspaper.

The trial coincides with an escalating dispute with Germany over the arrest in Turkey of 10 rights activists, including one German, as part of a crackdown since last year's attempted coup against Erdogan.

Turkish prosecutors are seeking up to 43 years in jail for newspaper staff accused of targeting Erdogan through "asymmetric war methods".

"I am not here because I knowingly and willingly helped a terrorist organisation, but because I am an independent, questioning and critical journalist," one of the defendants, columnist Kadri Gursel, told the court.

Gursel, who, along with editor Murat Sabuncu and other senior staff, has been in pre-trial detention for 267 days, was prevented from hugging his son in the courtroom by security guards, the newspaper said on its website.

The 324-page indictment alleges Cumhuriyet was effectively taken over by the network of U.S.-based cleric Fethullah Gulen, blamed for the failed putsch last July, and used to "veil the actions of terrorist groups".

Gulen has denied any involvement in the coup.

The newspaper is also accused of writing stories that serve "separatist manipulation".

Other defendants include Ahmet Sik, who once wrote a book critical of Gulen's movement. Former editor Can Dundar, who is living in Germany, is being tried in absentia.

The newspaper has called the charges "imaginary accusations and slander". Social media posts comprised the bulk of evidence in the indictment, along with allegations that staff had been in contact with users of Bylock, an encrypted messaging app the government says was used by Gulen's followers.

'DE FACTO COALITION'

"According to the government, everyone in opposition is a terrorist, the only non-terrorists are themselves," Filiz Kerestecioglu, a member of parliament from the pro-Kurdish HDP opposition party, told reporters ahead of the trial.

Gursel, the columnist, denied he had links to Gulen's movement, saying he had in the past revealed ties between Erdogan's AK Party and the Gulen movement.

Erdogan has his roots in political Islam and was an ally of the cleric until a public falling-out in 2013.

"I exposed the current government's de facto coalition with this group and I foresaw the harm that this sinister cooperation would do to the country," he told the court.

Rights groups and Turkey's Western allies have complained of deteriorating human rights under Erdogan. In the crackdown since last July's failed coup, 50,000 people have been jailed pending trial and some 150,000 detained or dismissed from their jobs.

As part of the purge some 150 media outlets have been shut down and around 160 journalists are in jail, according to the Turkish Journalists' Association.

The crackdown has strained Turkey's ties with the European Union, but reaction from the bloc has been restrained because it depends on Turkey to curb the flow of migrants into Europe.

Europe's leading power, Germany, has stepped up pressure in recent days, threatening measures that could hinder German investment in Turkey and reviewing Turkish applications for arms deals.

Turkish authorities say the crackdown is justified by the gravity of the coup attempt, in which rogue soldiers tried to overthrow the government and Erdogan, killing 250 people, most of them civilians.