Some analysts have seen the 450-kilometre (280 mile) trek from Ankara to Istanbul led by Republican People's Party (CHP) leader Kemal Kilicdaroglu as a significant challenge to Erdogan but the Turkish strongman has regarded it with disdain.
The CHP leader pressed on with the march accompanied by thousands of supporters despite blistering heat with the mercury touching 37 degrees Celsius (98.6 degrees Fahrenheit).
He dismissed Erdogan's comments as "fitting for a dictator".
Kilicdaroglu began the march on June 15 after former journalist turned CHP lawmaker Enis Berberoglu was sentenced to 25 years in jail on charges of leaking classified information to a newspaper.
Marching without party insignia and simply a sign with the word "justice" in Turkish, he has been followed by thousands every day and plans to end the march on July 9 with a mass rally outside Berberoglu's prison in the Istanbul district of Maltepe.
"If you start protests to protect terrorists and those who support terrorism -- when it did not occur to you to take part in anti-terror demonstrations -- then you cannot convince anyone that your objective is justice," Erdogan said.
The president told a meeting of his ruling party that the line represented by the CHP "had gone beyond being a political opposition and taken on a different proportion."
Accusing the CHP of sympathising with Kurdish militants and the alleged mastermind of the July 15 failed coup, he said the road taken by Kilicdaroglu was "the way to Qandil and Pennsylvania".
The leadership of the outlawed Kurdistan Workers Party (PKK) is based in the Qandil mountain of northern Iraq while the alleged coup mastermind, the preacher Fethullah Gulen, is based in Pennsylvania. He denies the allegations.
'Our right to walk'
The march by Kilicdaroglu has rallied supporters concerned by the extent of the crackdown after the coup which has seen tens of thousands arrested and even more lose their jobs.
It has also allowed him to make an impact on Turkish political life after narrowly losing the April 16 referendum on expanding Erdogan's powers, where the CHP campaigned for a 'No' vote.
The opposition leader on day 17 of the march headed to the town of Sakarya from where he will have a walk of around 150 kilometres (90 miles) to Istanbul.
Speaking to an AFP reporter during a break in the march, Kilicdaroglu shrugged off Erdogan's comments and vowed to press on with the trek until the July 9 mass rally.
"These words are fitting for a dictator," he said. "(It) is our right according to the constitution to walk."
Thousands of supporters, forming a vast file along the road, had earlier carried a Turkish flag several hundred metres (yards) long as they embarked on the day's march.
A CHP spokesman said 10,000 people joined Kilicdaroglu on Saturday.
People were clearly suffering in the extreme, shadeless heat.
The marchers took a long midday break of six hours before preparing to finish the day's walk in the evening.
Activists tried to encourage participants to avoid party political rhetoric and to shout just one simple slogan: "Rights, law and justice".
Justice Minister Bekir Bozdag echoed Erdogan's attack on the CHP with a late night broadside on Twitter, saying the party had to show it was not on the side of the coup plotters.
"I'll ask this -- hey CHP! Are you on the side of the coup? Or on the side of the martyrs, the heroes and the nation? Whose side are you really on?" said Bozdag.
Kilicdaroglu said he was no longer encouraging more supporters to join the march, with logistics already challenged by its size which surprised even the CHP.
"His (Erdogan's) anger against us shows that we are right to make this march," Kilicdaroglu told AFP.
