ANKARA (Reuters) - Turkey’s pro-Kurdish opposition applied on Tuesday for its jailed presidential candidate to be released before next month’s snap election, saying the detention of Selahattin Demirtas jeopardized voter freedom.

FILE PHOTO: A supporter of the pro-Kurdish Peoples' Democratic Party (HDP) hangs posters of Selahattin Demirtas, the party's jailed former co-leader and the candidate for the upcoming presidential election, during a gathering in Istanbul, Turkey May 4, 2018. REUTERS/Murad Sezer/File Photo

This month, the Peoples’ Democratic Party (HDP) nominated Demirtas, who has been in prison for about 17 months on security charges and faces a jail sentence of up to 142 years if convicted, as its candidate in the June election.

Turkey’s High Electoral Board has approved his candidacy and Demirtas is running his presidential campaign from behind bars.

The HDP said it had filed an appeal for the release of Demirtas, the party’s former leader, saying the imprisonment of a candidate violated electoral law and jeopardized voter freedom.

Demirtas, a former human rights lawyer, has expanded HDP’s support beyond its traditional Kurdish base by appealing to secular, left-leaning Turks. He has also won support from some other opposition candidates, such as nationalist Meral Aksener.

“He is not someone who has been convicted,” Aksener, the head of the Iyi (Good) Party, told reporters. “Let’s say he is freed three months after elections, how will Turkey explain the competitive inequality during the campaigning period then?”

Her comments were notable as nationalists and pro-Kurdish politicians rarely find common ground in Turkey.

The presidential candidate from the main opposition Republican People’s Party (CHP), Muharrem Ince, has also called for the release of Demirtas.

The HDP commands only about 10 to 12 percent of support from the electorate, so Erdogan faces a bigger challenge from Aksener and Ince in the polls.

Aksener, a former interior minister, founded her Iyi Party after splitting with the nationalist MHP party, which backs Erdogan.

This election will herald the switch to a powerful executive presidency narrowly approved in a referendum last year.

Aksener said that if the anti-Erdogan alliance were to obtain a majority in parliament, it would immediately move to revert to a parliamentary system, taking necessary constitutional changes to a referendum as soon as possible.

Polls have indicated that a first round victory for Erdogan is unlikely, despite very limited media coverage for opposition candidates, raising prospects of a second round vote between the top two candidates from the previous round.

Turkish media is saturated with coverage of Erdogan and his ministers, with the president’s daily routine of two or three speeches being broadcast on all major channels, while opposition parties receive little to no coverage.