In his first interview with international media since being named as candidate by HDP on May 4, Selahattin Demirtas told Reuters opposition parties will face huge obstacles campaigning for votes.

"Demonstrations are banned, talking is banned, criticizing the government is banned, even defending peace is considered terror propaganda," he said.

"Hundreds of opposition journalists are arrested, dozens of TV and radio channels are closed. It is impossible for there to be fair elections in such an environment," Demirtas said in a hand-written response to questions submitted by Reuters to his lawyers.

Demirtas and the HDP face even greater challenges than other opposition parties in running against Erdogan and his longtime-ruling Justice and Development Party (AKP).

Demirtas has been in jail for a year and a half on security charges and faces up to 142 years in prison if convicted.

Announcing his candidacy on May 4, the party released images of a visibly thinner yet smiling Demirtas dressed in a white shirt and black trousers in the courtyard of his prison in the northwestern province of Edirne.

A human rights lawyer by training, he is one of Turkey's best-known politicians, winning votes beyond his core Kurdish constituency in the 2015 election to turn the HDP into the second-largest opposition party in parliament.

Now, he faces terror charges.

Prosecutors charge that Demirtas and hundreds of other detained HDP members are tied to the outlawed Kurdistan Workers Party (PKK).

The HDP denies the charges and Demirtas said he was unjustly jailed.

"There is no legal obstacle to my candidacy because I am not convicted," he said, adding it would be a "scandal and a crime" if the courts blocked him by convicting him.

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