Kemal Kilicdaroglu, the leader of the secular Republican People's Party (CHP), launched an unprecedented 450-kilometre (280 mile) trek on June 15 from Ankara to Istanbul in protest the arrest of one of his MPs.
Supporters have compared the trek of the slightly built, mustachioed 69-year-old with Indian independence leader Mahatma Gandhi's famous Salt March of 1930. But the government has dismissed the march as a bothersome stunt.
Tens of 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 sparked by the planned redevelopment of Gezi Park in the city.
'Grave injustices'
The CHP leader reached the outskirts of Istanbul on Friday and was joined by tens of thousands forming a vast file along the road despite blistering heat.
He was due on Sunday from 5:00 pm (1400 GMT) to walk the final three kilometres alone before joining the waiting crowds at a vast meeting space in the Istanbul district of Maltepe.
"Why do I walk? This 450-kilometre march has one goal: Justice," Kilicdaroglu said ahead of the meeting.
"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.
Kilicdaroglu launched the march from Ankara 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.
The rally ground is near Berberoglu's prison in the Istanbul district of Maltepe.
Carrying simple insignia emblazoned with the word "Justice" without any party slogans, he turned the march into a protest for those who feel unfairly treated in Turkey under Erdogan.
Kilicdaroglu has said he wants no party insignia, only "Justice" slogans and pictures of modern Turkey's founder Mustafa Kemal Ataturk at the Maltepe rally.
Preparations were in full swing for the rally, with a vast stage set up with the Turkish word "Adalet" (Justice) plastered in giant letters across the top.
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.
Kilicdaroglu has strongly condemned the failed coup bid -- blamed US-based preacher Fethullah Gulen, who denies the charges -- but has been bitterly critical of the scope of the state of emergency.
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 sparking a new uproar among rights advocates.
High security
The opposition chief has dressed every day modestly in a white shirt, dark trousers, with a hat to protect him from the sun. He rested at night in a caravan.
"He is surprisingly vigorous," his party's Istanbul lawmaker Sezgin Tanrikulu told AFP.
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."
The authorities have not impeded the walk's progress, with police providing security every day.
Istanbul governor Vasip Sahin said 15,000 police had been assigned to assure security at the Maltepe meeting.
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.
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