The Peoples' Democratic Party (HDP) had agreed Demirtas as its candidate and his campaign would be launched in simultaneous rallies in Istanbul and the majority Kurdish city of Diyarbakir on Friday afternoon, the party said.
Turkey is due to vote in presidential and parliamentary elections on June 24 after snap polls were called by President Recep Tayyip Erdogan one-and-a-half years ahead of schedule.
Demirtas, 45, was a candidate against Erdogan in August 2014 presidential elections and led HDP into parliament for the first time in June 2015 polls.
Dubbed the "Kurdish Obama" by admirers after the rhetorically-gifted former US president, Demirtas stood out in Turkish politics with impassioned speeches and a radiant charisma.
But he was arrested in November 2016 in a crackdown that followed the July failed coup and put on trial in several cases, notably on charges of links to the outlawed Kurdistan Workers' Party (PKK).
If convicted, he faces up to 142 years in prison.
The HDP replaced Demirtas as its co-leader in February this year after he indicated his political career was over while in prison.
It was not immediately clear what had prompted the change of heart and how he can register as a candidate, let alone campaign, while in jail.
The new co-leaders of the HDP, Pervin Buldan and Sezai Temelli, urged supporters to come out on Friday and celebrate the campaign launch.
"We are coming together to share our joy in Selahattin Demirtas' candidacy for president," they said in a statement. "We can already see a bright future."
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