ISTANBUL (AP) — Turkey's main opposition party has started a 400-km (250-mile) march from the capital to an Istanbul prison to protest the imprisonment of one of its lawmakers.

The leader of the pro-secular Republican People's Party, Kemal Kilicdaroglu, said Thursday he is seeking justice. He called the march after parliamentarian Enis Berberoglu was convicted to 25 years in prison for revealing state secrets.

With thousands gathered in protest, Kilicdaroglu said: "Everyone needs to defend the independence of the judiciary and justice in this country."

The guilty verdict for Berberoglu is part of a case that stems from a 2015 story by the Cumhuriyet newspaper suggesting Turkey's intelligence service had smuggled weapons to Islamist rebels in Syria.