Hundreds of Turkish human rights activists, academics and journalists have been prosecuted in recent months in what critics say is a wide-ranging crackdown on free speech.

The Islamist president, Recep Tayyip Erdogan, has attracted international criticism as tensions increase between his government, Syria and the Kurds.

The European Union labels it a "serious backsliding" on freedom of expression and the US says there could be a "chilling effect" on legitimate political debate.

One of the country's oldest newspapers, Cumhuriyet, is locked in a battle with Mr Erdogan.

"They want to turn Turkey into a country with one single voice, a country that obeys and accepts without doubt anything Erdogan says," said acting editor Murat Subuncu,.

In May last year the paper published evidence that the government was shipping weapons to rebels in Syria.

The story highlighted concern about Turkey's support for Islamist rebels amidst the rise of the Islamic State extremists.

"Anything that Erdogan dislikes is a taboo in Turkey particularly. And this is a big taboo because revealing the transfer of arms to Syria creates trouble regarding the role Turkey is playing in the region," Mr Subuncu said.

Journalists facing life in prison

After the story was published, Mr Erdogan vowed that the authors would pay a heavy price and the editor, Can Dundar, and another staffer, Erdem Gul, were arrested.

This month, prosecutors demanded life sentences for both, accusing them of aiding terrorists and publishing state secrets.

Mr Subuncu said 21 of the paper's journalists were being investigated or prosecuted and 14 Turkish journalists were now in prison.

"People start to censor themselves when they see such prominent journalists arrested," he said.

"We are standing against this censorship but there aren't many of us left."

Human rights watchers are also alarmed by Turkey's battle with the Kurdish Workers' Party, the PKK, in the south-east of the country.

The bomb blast at the pro-Kurdish peace rally in October last year was captured on camera. ( Dokuz8 Haber News Agency: @dokuz8haber )

Last year, when a pro-Kurdish peace rally was bombed in Ankara, the government blamed the Kurds, then Islamic State.

But many, like veteran human rights campaigner Eran Keskin, claimed Mr Erdogan's government had a hand in the attack.

For that claim, she is being prosecuted for insulting the president.

She is one of more than 130 people now thought to be facing that charge.

Pro-Kurdish activists targeted

Ms Keskin said Mr Erdogan was running a policy of fear.

"I think Erdogan has been successful in getting public support by frightening people. He applied fear as a policy and has been rewarded," she said.

Ms Keskin's career has spanned many changes of government.

She served six months in jail in the mid-90s for using the word Kurdistan, and she is still fighting a 10-month prison term for accusing the authorities of killing a Kurdish boy in 2004.

In fact, she has weathered more than 100 judicial proceedings against her, since well before Mr Erdogan came to power.

But she said today's ruler was proving especially censorious.

"This is a type of president we have never seen before in Turkey," she said.

"President Tayyip Erdogan is a person who cannot stand any thought different than his thoughts and he is doing this very brazenly."

Further highlighting the government's crackdown on dissent, earlier this year at least 19 academics were arrested after they signed a petition denouncing military operations against the Kurds.

Amnesty International's Ruhat Sena Aksener said it was an ominous sign for human rights in Turkey.

"It's a direct message to the public in fact that means if you say some words that are not pro-state policies, or if you criticise something that we do not like, you will be punished," she said.

"That's a very strict message."

Watch Matt Brown's report on Lateline tonight at 9:30pm (AEDT) on ABC News 24 or 10:30pm on ABC TV.