Demirtas on Wednesday issued his first, and possibly only, audio message of the campaign where he will challenge President Recep Tayyip Erdogan, delivered in a telephone call received by his wife Basak.
The charismatic ex-leader of the pro-Kurdish Peoples' Democratic Party (HDP) has been jailed since November 2016 on charges of links to outlawed Kurdish militants and is now in a prison in the northwestern region of Edirne.
The HDP produced a slick video of the message, which begins with Basak Demirtas welcoming family members to their home in the southeastern city of Diyarbakir.
Her mobile rings and it's her husband. The phone is put on loudspeaker.
"Hello my darling, how are you?" she says, and after some brief pleasantries lets him make the around five minute campaign speech.
"Sadly, Turkey has been transformed into a semi-open prison. They are trying to create a society based on fear and reign though fear," he declared.
Demirtas, speaking confidently and without interruption from an apparently prepared text, described himself as a "political hostage".
But he also urged optimism among the HDP faithful. "Demirtas is not the man who is in a cell in Edirne. It's you. Have confidence in yourselves."
At the end of his address, the family members, who had sat in respectful silence, erupted into applause, ululating and chanting "Selo (a short form of Selahattin) for president".
As well as his incarceration, Demirtas has had to contend with being all but ignored by state television, which broadcasts every moment of Erdogan's speeches live.
But he has sought to campaign as best he can, notably posting regular tweets sent via his lawyers and also making numerous media interviews conducted in the same way.
Demirtas until his jailing was seen as the only Turkish politician with the rhetorical skills and charisma to match those of Erdogan.
Erdogan has in recent days upped his campaign attacks on Demirtas, accusing him of being a "terrorist" responsible for the deaths of dozens by calling protests in October 2014 that turned violent.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)