Turkey’s main opposition party has begun a march from the country’s capital, Ankara, to its largest city, Istanbul, to protest against the imprisonment of an MP who was sentenced to 25 years in jail for allegedly leaking information to the press.

“We are facing dictatorial rule,” said Kemal Kılıçdaroğlu, the leader of the Republican People’s party (CHP), as he set out on the march. “We don’t want to live in a country where there is no justice. We are saying enough is enough.”

Hundreds of protesters appeared to have joined the march, carrying banners that said “adalet” or “justice” as they set out on the 280 mile (450km) trek that will take them to Maltepe prison, where Enis Berberoğlu has been incarcerated.

A court in Istanbul on Wednesday sentenced Berberoğlu, a CHP MP, to 25 years in prison over his alleged leaking of state secrets to the daily opposition newspaper Cumhuriyet.

The story concerned an incident in 2014 when the Turkish gendarmerie intercepted vehicles bound for the Syrian border belonging to the National Intelligence Organisation (MIT) that were ostensibly carrying humanitarian aid but were instead carrying arms.

It is the first time a CHP MP has been the target of a judicial sentence since parliament voted last year to lift the immunity of members of parliament. The measure was primarily aimed at the pro-Kurdish party in the legislature but has now targeted the CHP, which controls just over a third of the seats in the assembly and voted to lift the immunity.

Demonstrators shout slogans during the march. Photograph: Murad Sezer/Reuters

Two journalists from Cumhuriyet are also standing trial in the MIT trucks case – Can Dundar, its former editor-in-chief, and Erdem Gul, the newspaper’s Ankara bureau chief. They are accused of colluding with the network of Fethullah Gulen, an exiled preacher whose movement is widely believed in Turkey to have orchestrated a coup attempt last July.

Opposition members and rights activists have long said that the crackdown following the failed putsch has gone beyond the perpetrators and is aimed at stifling dissent. While tens of thousands of people have lost their jobs or been detained over alleged Gulen links, the imprisonment of a lawmaker from the mainstream opposition is a fresh escalation in the campaign.

On Thursday a court also sentenced a Turkish UN war crimes judge to seven-and-a-half years in prison, a decision condemned by his tribunal in the Hague as a breach of diplomatic norms.

Judge Aydin Akay was detained in September as part of a crackdown on the judiciary following the coup attempt. Akay is a judge at a Netherlands-based UN court handling final appeals and cases from the Rwanda and Yugoslavia war crimes tribunals. His detention has delayed those cases.

The tribunal’s president, Theodor Meron, said Akay’s arrest, “detention and legal proceedings against him are inconsistent with the assertion of his diplomatic immunity by the United Nations”, as well as a court order by the tribunal to release him in January.