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.
Kilicdaroglu said the "second coup" came after the failed July 15 coup. Erdogan declared a state of emergency on July 20 that has since seen about 50,000 people arrested and over 100,000 lose their jobs.
"The outcome (of July 15) was used to make the second coup on July 20," Kilicdaroglu told AFP on the second day of his 450-kilometre (280 miles) trek to Istanbul which is expected to take over three weeks.
The initial three-month state of emergency has since been renewed three times, amplifying concerns about human rights in the country.
The CHP leader, dressed in a blue shirt, baseball hat and sport shoes, said the march was a challenge to injustice and was needed "because there is no independent judiciary".
"The judiciary largely await instructions from Erdogan and they give decisions based on these instructions," he said, as supporters brandished placards with a single word, "justice".
'Recalls Gandhi'
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."
The Hurriyet daily and other papers recalled the famous 24-day barefoot "Salt March" of Indian independence leader Mahatma Gandhi in 1930.
CHP supporters like to compare Kilicdaroglu to Gandhi due to a physical resemblance, softly spoken manner and avowed desire to solve conflicts peacefully.
"I am well," he said. "Because we are right, we feel no fatigue."
"I said I would walk for justice, I said I would walk alone but today several citizens have taken part," said Kilicdaroglu who has been accompanied by thousands of supporters so far.
'Solution not on streets'
Prime Minister Binali Yildirim on Friday 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".
"Seeking solutions on the street is at the very least irresponsible," Yildirim said, adding there could be "provocations".
Berberoglu's case, which sparked the march, relates to the 2015 publication by the opposition Cumhuriyet newspaper of images purportedly showing the Turkish intelligence service seeking to transport arms over the border to Syria.
A dozen MPs from the pro-Kurdish Peoples' Democratic Party (HDP) had already been arrested under the emergency as well as thousands of party officials.
But this was the first time an MP from the CHP, the main secular opposition founded by first president Mustafa Kemal Ataturk, has been held.
'Controlled coup'
Stepping up his challenge to Erdogan, Kilicdaroglu controversially claimed the government knew about the July 15 coup bid but did not prevent it from happening.
He has previously referred to the failed putsch -- which Ankara blames on the US-based preacher Fethullah Gulen -- as a "controlled coup".
Erdogan at the time accused him of a "big lie" while Yildirim accused Kilicdaroglu of insulting the 249 people killed in the coup.
Kilicdaroglu defended the solidarity he showed the president in the aftermath of the coup bid -- which extended to even visiting Erdogan's palace he had once called illegal and vowed to never set foot in.
"If there is a similar situation, we would continue to be resolutely against any coup in the same way," he said.
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