Kemal Kilicdaroglu, the leader of the secular Republican People's Party (CHP), launched an unprecedented 450-kilometre (280 mile) trek on June 15.
Kemal Kilicdaroglu, the leader of the secular Republican People's Party (CHP), launched an unprecedented 450-kilometre (280 mile) trek on June 15 in Ankara in protest at the arrest of one of his MPs.
Supporters compared the trek with Indian independence leader Mahatma Gandhi's famous Salt March of 1930 but the government has dismissed it as a bothersome stunt.
Carrying a simple insignia emblazoned with the word "Justice" without any party slogans, Kilicdaroglu initiated the march after his party's lawmaker Enis Berberoglu, a former journalist, was sentenced to 25 years in jail on charges of leaking classified information to a newspaper.
"Why do I walk? This 450-kilometre march has one goal: Justice," said the CHP leader, who reached Istanbul on Friday and was joined by tens of thousands forming a vast file along the road despite blistering heat.
"They ask 'Can we seek justice on the road?' Yes we can. If there are grave injustices and illegalities in your country and if your country's courts are incapable of delivering justice, you will stand up and hit the road," he said in a statement to AFP.
"That's what I am doing now. I have one motto: JUSTICE."
About 50,000 people have been arrested under Turkey's state of emergency, imposed after last July's failed coup, and another 100,000 have lost their jobs, including teachers, judges, soldiers and police officers.
In the latest crackdown, Turkish police on Wednesday detained Amnesty International's Turkey director and other activists on charges of membership in a terror group, which sparked uproar among human rights advocates.
Thousands of people are expected to show up for the rally, which could be one of the biggest opposition protests seen in Istanbul since the mass 2013 demonstrations against Erdogan's rule.
Kilicdaroglu, who will address his followers in a mass rally near Berberoglu's prison in the Istanbul district of Maltepe, will on Sunday walk the final section of the march alone.
"I will walk to the rally ground alone," he told reporters.
"I do not want anything in the rally apart from (Turkish) flags, Justice banners and (Turkey's founder) Ataturk's posters," he said.
The opposition chief rested at nights in a caravan throughout his journey, and witnesses said the 69-year-old was walking quite fast.
"He is surprisingly vigorous," his party's Istanbul lawmaker Sezgin Tanrikulu told AFP.
Commenting on his eating habits, he said Kilicdaroglu "never has heavy meals."
The government has regarded the justice march with disdain.
Erdogan has accused Kilicdaroglu's party of siding with terrorism while Prime Minister Binali Yildirim said Friday: "(The march) has started to become boring. This should come to an end after the rally."
Authorities, however, have not impeded the walk's progress, with police providing security every day.
Turkish police this week detained six suspected members of the Islamic State extremist group planning a bomb attack on the march. But the CHP said it was a routine operation, and was not related to the justice march.