The biggest media trial in Turkey's history has begun in what human rights groups say is an attempt by the government to intimidate the press and punish pro-Kurdish activists.

A total of 44 Kurdish journalists appeared in court in Istanbul on various terrorism charges, including accusations that they have supported the KCK, an illegal pan-Kurdish movement that includes the PKK, the armed Kurdistan Workers' party. Of those, 36 have been in pre-trial detention since December.

The hearing was delayed after the defendants made an attempt to defend themselves in Kurdish, their mother language, a request denied by the judge. Twelve of the defendants are said to have led a terrorist organisation and 32 are accused of being members of a terrorist organisation. Prosecutors have demanded prison sentences ranging from seven and a half to 22 and a half years.

The contentious case comes amid an escalation of Turkey's 28-year-old Kurdish insurgency, with renewed clashes between the PKK and Turkish security forces. Over the past 14 months, the country has seen its worst violence since the PKK's leader, Abdullah Öcalan, was captured and jailed in 1999. Since June 2011, at least 708 people have been killed, according to the Brussels-based International Crisis Group. The victims include 405 PKK fighters, 209 soldiers and police, and 84 civilians, it said.

Meanwhile, a peaceful initiative by Turkey's prime minister, Recep Tayyip Erdoğan, and his ruling Justice and Development party (AKP) to improve Kurdish rights has fizzled out. Instead, thousands of non-violent Kurdish politicians and sympathisers have been arrested and charged with terrorism offences. The journalists are the latest group to go on trial, activists say.

"This is bad for Turkey's international image," said Hüseyin Bagci, of Ankara's Middle East Technical University's international relations department. Bagci described Erdoğan's Kurdish political initiative – unpopular with many Turks – as dead, but said the government remained divided over how to deal with the worsening insurgency, with no clear strategy.

Human rights groups have repeatedly criticised the Turkish government for the prosecution of pro-Kurdish politicians, activists and journalists who exercise their right to freedom of expression.

Andrew Gardner, Turkey researcher of Amnesty International, said: "[This] prosecution forms a pattern where critical writing, political speeches and participation at peaceful demonstrations are used as evidence of terrorism offences."

More than 100 journalists are currently in jail in Turkey, more than in Iran or China. Many of them work for Kurdish media outlets. About 800 more face charges and many journalists have been fired or have quit their jobs because of direct or indirect pressure from the Turkish government.

In a recent speech, the interior minister, Idris Naim Sahin, compared writers and journalists to PKK fighters, saying that there was "no difference between the bullets fired in [the Kurdish south-east] and the articles written in Ankara".

The government maintains that none of the journalists on trial have been arrested for their work as members of the press. However, the 800-page indictment includes charges for "denigrating the state" against one journalist who wrote about sexual harassment at Turkish Airlines. Özlem Agus, a reporter for the pro-Kurdish Tigris News Agency (DIHA), was singled out for bringing to light sexual abuse of minors in the Pozanti prison in Adana. Other offending articles include interviews with the pro-Kurdish Peace and Democracy party (BDP) leader Sebahattin Demirtas, and reports on casualties in the fights between the PKK and Turkish armed forces.

"All of the defendants are on trial for doing their jobs," the defence lawyer Meral Danis Bektas said. "A free press and freedom of expression are cornerstones of democracy. Without them, democratic political participation becomes impossible. Erdoğan now openly threatens journalists or dictates [what to write]. This attitude creates a terrible climate for press freedom."

A report by the International Crisis Group to be published on Tuesday blames both sides for the worsening situation. It says the government needs to "reform oppressive laws that jail legitimate Kurdish politicians" and to "make amends" for the excessive behaviour of its security forces. But it adds: "The Kurdish movement, including PKK leaders, must abjure terrorist attacks and publicly commit to realistic political goals. Above all, politicians on all sides must legalise the rights most of Turkey's Kurds seek, including mother-language education, an end to discriminatory laws, fair political representation and more decentralisation."

The report also claims Ankara has "zigzagged" on its commitments to Kurds' rights. At times it has given "positive signals" including scheduling optional Kurdish lesson in schools. "At others, they appear intent on crushing the PKK militarily, minimise the true extent of fighting, fail to sympathise with Kurdish civilian casualties, openly show their deep distrust of the Kurdish movement, do nothing to stop the arrest of thousands of non-violent activists and generally remain complacent as international partners mute their criticism at a time of Middle East turmoil."

Since 2009, 8,000 pro-Kurdish politicians, lawyers, academics, writers and members of the media have been arrested on terrorism charges.

The new media trial "is clearly political," said the investigative journalist Ertugrul Mavioglu, who faced terrorism charges, dropped last December, for interviewing the KCK's leader Murat Karayilan, who operates from a base in northern Iraq.

Mavioglu said: "The government wants to set an example, it wants to intimidate. Journalists are being told: 'There are limits on what you are allowed to say.'"

• This article was amended on 11 September to correct the English translation of the BDP's name, from the Freedom and Democracy party to the Peace and Democracy party