Republican People's Party (CHP) leader Kemal Kilicdaroglu called the march after former journalist turned CHP lawmaker Enis Berberoglu was sentenced to 25 years in jail on Wednesday for leaking classified information to a newspaper.
In his biggest challenge yet to the authorities, the leader of Turkey's second-largest party is marching to the prison in the Istanbul district of Maltepe where Berberoglu is being held, inviting supporters to accompany him.
"The results (of July 15) were used to make the second coup on July 20," Kilicdaroglu told AFP on the second day of his 450-kilometre trek to Istanbul.
The initial three-month state of emergency has since been renewed three times, amplifying concerns about human rights in the country.
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The CHP leader, dressed in a shirt, baseball hat and sport shoes, said the march was a challenge to injustice and was needed "because there is no independent judiciary".
Security forces often clamp down on protests in Ankara and Istanbul, but Kilicdaroglu's march has taken place unhindered so far, albeit with heavy security.
Kilicdaroglu said he did not believe the government would have the "courage" to stop the march or detain him "because the march is within the limits of the law".
But he warned: "If detentions or arrests happen, there would be heavy consequences."
Prime Minister Binali Yildirim today criticised Kilicdaroglu's actions, saying justice "cannot be found on the streets" and "we must respect courts' decisions even if we don't like them".