Ankara (AFP) - A Turkish court on Wednesday sentenced a Republican People's Party (CHP) lawmaker to 25 years jail for leaking classified material to the press, the first time a main opposition party deputy has been locked up in an ongoing crackdown.

CHP lawmaker Enis Berberoglu had not been detained during his trial but after the verdict was announced he was immediately placed under arrest at the Istanbul criminal court, state-run Anadolu news agency said.

A dozen lawmakers from the pro-Kurdish Peoples' Democratic Party (HDP) have been arrested under the state of emergency that followed last year's failed coup.

But this is the first time a deputy from the CHP -- the main secular opposition founded by Turkey's first president Mustafa Kemal Ataturk -- has been held.

The case relates to the controversial publication by the Cumhuriyet daily in 2015 of images purportedly showing the Turkish intelligence service seeking to transport arms over the border to Syria.

Video footage showed police opening crates of weapons and ammunition on the back of trucks which Cumhuriyet claimed belonged to the Turkish Intelligence Organisation (MIT).

The story infuriated Turkish President Recep Tayyip Erdogan who at the time warned Cumhuriyet's former editor in chief, Can Dundar, he would "pay a heavy price".

Dundar was last year handed a five-year-and-10-month jail term in the case. He has since fled to Germany.

Berberoglu was accused of handing over the images to Dundar. He was given a life sentence but this was reduced to 25 years by the court.

- 'State bullying of CHP' -

The CHP immediately held an emergency meeting after the verdict by the Istanbul court.

After the meeting, CHP leader Kemal Kilicdaroglu said "we absolutely, certainly do not accept this" decision, which he accused of being influenced "by the palace" referring indirectly to Erdogan.

"None of them are judges, they do not administer justice," referring to those deciding on Berberoglu's case.

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"Those who made this decision will live under the heaviness of its unjust nature. We have devoted our lives to democracy, to justice. We want justice in this country, democracy... freedom of thought," Kilicdaroglu told reporters in Ankara.

The CHP leader said he would hold a march on Thursday in the heart of Ankara at 11 am (0800 GMT) "holding only one poster with 'justice' written".

The party's deputies also walked out of Wednesday's parliament session in protest, Turkish television said.

Before his arrest, Berberoglu said he had been "victimised" and those responsible for the injustice "should be ashamed".

Berberoglu's wife, Oya, was quoted by Dogan news agency saying that her husband's morale was "good" but added: "I am protesting against this massive injustice. Enis is going to prison but it is the CHP who is on trial here."

The head of the CHP's parliamentary faction Engin Altay slammed the verdict as "state bullying" of his party.

"This decision is intimidation of the opposition. This decision is intimidation of those displeased with the (ruling) Justice and Development Party (AKP), those unhappy with anti-democratic practices," Altay said in Istanbul.

The AKP was co-founded by Erdogan, who in April won a controversial referendum giving him greater powers that opponents fear will lead to one-man rule.

- 'Punishing journalism' -

Cumhuriyet's Ankara bureau chief Erdem Gul, who is himself appealing his five-year sentence for revealing state secrets, hit back at the court's ruling.

"This is a trial trying to prevent journalism in this country. Journalism is really in a grave situation," Gul said.

"We are seeing an injustice being done. We are sad. At the same time, this is an arrest of an MP but also this is... a case against journalism, an attempt to punish journalism," he added.

Turkey ranks 155 on the latest RSF world press freedom index, below Belarus and the Democratic Republic of Congo, and a drop of four places from its 2016 ranking.

Western countries and human rights activists have expressed concern over what they say is a deteriorating situation for journalists in the country.

According to the P24 press freedom website, there are 167 journalists behind bars in Turkey, most of whom were detained as part of the state of emergency imposed after last July's coup attempt.

However, Turkey insists journalists have not been detained because of their work and all press freedoms exist for those observing the law.