On Thursday, thousands of demonstrators set off on the protest march, which is being led by the leader of the Republican People's Party (CHP), Kemal Kilicdaroglu, Presstv.ir reported.
Turkish media reports said the march would take two weeks, covering approximately 15 to 20 kilometers each day.
Kilicdaroglu, who is reportedly being escorted by his security detail, stressed that he wants justice, democracy and freedom in Turkey.
"Let the whole world hear, we are facing a dictatorial regime in Turkey, in our own land," media outlets quoted the opposition leader as saying.
The angry demonstrators also held banners and shouted pro-democracy slogans.
Kilicdaroglu called the march after Enis Berberoglu, a deputy chairman for the CHP, was sentenced to 25 years in prison on Wednesday for revealing state secrets.
Berberoglu was convicted for his role in leaking secret documents to a newspaper showing the country's National Intelligence Organization (MIT) shipped weapons to foreign-backed Takfiri terrorists in Syria.
Istanbul’s 14th Heavy Penal Court handed down the sentence to Berberoglu for releasing secret documents with the purpose of political or military espionage.
Berberoglu, a former journalist, was arrested in the courthouse after the hearing. He will remain under arrest while waiting for the appeal process to conclude.
Following the court verdict, Kilicdaroglu tweeted, "In this country, the punishment for covering the news of a truck filled with weapons heading to terror groups is 25 years in prison but illegal arm shipments are allowed!"
Back in May 2015, Cumhuriyet daily posted on its website footage showing Turkish security forces in early 2014 intercepting a convoy of trucks carrying arms for the militants in Syria.
The paper said the trucks were carrying some 1,000 mortar shells, hundreds of grenade launchers and more than 80,000 rounds of ammunition for light and heavy weapons.
Ankara denied the allegation and claimed that the trucks had been carrying humanitarian aid to Syria. However, Berberoglu defended the video, saying it was genuine.
The incident triggered a huge controversy in Turkey, with many bashing the government for explicitly supporting terrorism in neighboring Syria.
Cumhuriyet’s former editor-in-chief Can Dundar and Ankara bureau chief Erdem Gul were among other defendants in the case.
Last year, Dundar and Gul were sentenced to at least five years in jail for revealing what was said to be state secrets. The prosecutor is now seeking an additional 10 years in prison for the two over the report on MIT trucks.
Separately, Berberoglu , Dundar and Gul are being tried for "aiding a terror organization without being members," referring to the network of US-based cleric Fethullah Gulen, Whom Turkey blames for a failed coup in July last year.
Gulen has denied the charges of having masterminded the coup.
The prosecutor believes Gulen's network to be the source of the leaked images.
After the abortive coup in July 2016, authorities arrested 50,000 people and sacked or suspended 120,000 others from a wide range of professions, including soldiers, police, teachers, and public servants, over alleged links to Gulen.
Berberoglu is the first legislator from the CHP to be imprisoned since a constitutional amendment stripped parliamentary immunities last year.