Thousands of protesters gathered in central Ankara on Thursday after the main opposition leader called for a march for justice following the jailing of one of his party's lawmakers.
Kemal Kilicdaroglu, head of the Republican People's Party (CHP), announced the demonstration Wednesday after Enis Berberoglu was sentenced to 25 years for giving a newspaper information on the transportation of arms to Syria.
"We do not want to live in a country where there is no justice," Kilicdaroglu told reporters in Guvenpark in Ankara.
"We want to live in peace in our own country, like any free or civilised country."
Carrying banners that read "Justice", and waving Turkish flags, crowds demonstrated as 68-year-old Kilicdaroglu set off at the head of the 425 kilometre (265 mile) march from the capital Ankara to the Istanbul jail where Berberoglu is being held.
CHP officials said the journey could take 20 days.
Kilicdaroglu was accompanied by CHP executives and supporters as well as family members.
However, he said the demonstration was not a party political event. "Everyone who wants justice has to support this march," he said. "There cannot be justice in a country where it is prisons are overcrowded."
Berberoglu was convicted of revealing state secrets by passing photographs and video to the Cumhuriyet newspaper concerning the search of trucks organised by the National Intelligence Organisation en route to Syria in January 2014.
Cumhuriyet Editor-in-Chief Can Dundar and the newspaper's Ankara bureau chief Erdem Gul were convicted in May last year of publishing state secrets.
Berberoglu is the latest CHP deputy to be imprisoned. About a dozen lawmakers from the Peoples' Democratic Party (HDP) are currently jailed, mostly awaiting trial over alleged links to the PKK terror group.
In May last year, parliament voted to strip lawmakers facing trial of their parliamentary immunity.