Turkish main opposition leader Kemal Kilicdaroglu is preparing to hold a mass rally in Istanbul, marking the end of his 450km protest march from capital Ankara to the metropolis.

He started the "March for Justice" 25 days ago in protest of the conviction of Enis Berberoglu, an MP with his Republican People's Party (CHP), and other recent arrests in Turkey.

Kilicdaroglu has been followed by thousands of supporters along the way, including CHP deputies.

Berberoglu was sentenced to 25 years in prison last month for allegedly revealing state secrets to journalists on the transportation of arms to Syria.

Kilicdaroglu said in an opinion article published in the New York Times on Friday that his supporters and he were marching for "democracy, justice and freedom from fear and authoritarian rule in Turkey".

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He wrote: "If democracy and rule of law are suspended, if people are afraid to freely express their views, if legislators are in prison rather than in the parliament, if the courts are incapable of serving justice, we stand up and call for justice with our words, with our bodies on the streets."

Turkish President Recep Tayyip Erdogan recently slammed the march, saying that the CHP is collaborating with "terrorist" groups and forces that incite such groups against Turkey.

"If you are launching a march for terrorists and for their supporters, something which you have never thought about doing against terrorist groups, you can convince no one that your aim is justice," he said.

"This situation, which anyone who in their right mind can see clearly, is part of traps set up in Syria, Iraq, the Gulf, and Europe against our country," he added.

CHP expects masses

On the 25th day of his march, Kilicdaroglu is expected to walk from the neighbourhood of Dragos to Maltepe in Istanbul, where he will hold a rally that his party says it expects over a million people to attend.

Berberoglu was convicted of revealing state secrets by passing images to a newspaper appearing to show the search of National Intelligence Organisation (MIT) trucks en route to Syria in January 2014.

He is the first CHP deputy to be imprisoned in recent years, while mass arrests have targeted MPs of the pro-Kurdish People's Democracy Party (HDP) in recent months, including its leaders.

Turkey has been in a state of emergency after a coup attempt last summer which killed around 300 people and led to arrests and purges targeting tens of thousands of civil servants as well as journalists and opposition activists.