The head of Republican People's Party (CHP), the main opposition party in Turkey, is due to address a mass rally in Istanbul at the end of an almost month-long protest march held in order to voice dissent against the detention of a party's lawmaker, in a rare challenge to Turkish President Recep Tayyip Erdogan.

Kemal Kilicdaroglu, 68, started his protest march from Ankara on June 15, and after covering some 450 kilometers reached the outskirts of Istanbul on Friday. At 05:00 p.m. local time (1400 GMT) on Sunday he set to walk the final three kilometers alone to reach Istanbul district of Maltepe, where tens of thousands of supporters convened at a vast meeting space, waiting for his speech.

    Why do I walk? This 450-kilometer march has one goal: Justice, he said ahead of the mass 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, Kilicdaroglu added.

He said he walked for absent justice, adding that the march was only a first step and a rebirth. He also vowed that people would bring justice to the country, describing the current era as a dictatorial era.

Turkey's main opposition Republican People's Party (CHP) leader Kemal Kilicdaroglu, bottom center, walks on the final stage of his 25-day long protest, dubbed Justice March, against the detention of the party's lawmaker Enis Berberoglu, in Istanbul, Turkey, July 9, 2017. (Photo by Reuters)

Kilicdaroglu began his march a day after a court in Ankara sentenced former journalist turned CHP lawmaker Enis Berberoglu to 25 years in prison for his alleged role in leaking secret documents to a newspaper showing the country's National Intelligence Organization (MIT) shipped weapons to foreign-backed Takfiri terrorists in Syria.

Berberoglu is currently held at a prison in the same district, where Kilicdaroglu is set to address a massive crowed.

The Turkish government has already dismissed March for Justice as an irritation, and back in June 18, Erdogan said that actions like the march by Kilicdaroglu would bring no good for Turkey, saying the opposition leader should not be surprised if legal proceedings were opened.

However, Kilicdaroglu responded that he was undeterred by Erdogan's threats that the action might lead to legal proceedings against him.

 They want to provoke us but we will not give in! They want to threaten us with the courts and he (Erdogan) calls out the judges, the prosecutors. We are walking for justice, not against justice, he further said at the time.

People chant slogans as Turkey's main opposition Republican People's Party (CHP) leader Kemal Kilicdaroglu (not pictured) walks on the final stage of his 25-day long protest, dubbed Justice March, against the detention of the party's lawmaker Enis Berberoglu, in Istanbul, Turkey, July 9, 2017. (Photo by Reuters)

Turkey has seen a surge in political fighting since a failed coup attempt on July 15 last year. The opposition has constantly criticized a crackdown launched since the coup which has seen more than 40,000 people arrested and over 100,000 discharged from their jobs. Kilicdaroglu even once accused Erdogan of being the July 20 coup plotter, saying he had used the deadly coup by military renegades to consolidate his rule in Turkey.

Tensions rose again on April 16 when voters endorsed changes to Turkey's constitution and gave Erdogan more powers as president. The CHP has censured Erdogan's narrow victory in the referendum while warning that Erdogan would slide Turkey into a one-man rule system.