Demirtas, 45, is the ex-chief of the leftist Peoples' Democratic Party (HDP) is running against President Recep Tayyip Erdogan in the June 24 polls.

He was jailed in November 2016 alongside 11 HDP lawmakers over alleged links to outlawed Kurdish militants. Demirtas faces multiple legal cases.

The party applied to the country's top court early Friday, an HDP official told AFP, after its previous requests for Demirtas' release were rejected this month.

"The Constitutional Court must meet to discuss this case and, taking into account electoral rights and eligibility, order his release at once," HDP co-chief Sezai Temelli said in the northwestern province of Edirne.

"This country will be dragged into deepening crises if the opposite of this happens," Temelli warned during a gathering three kilometres (1.9 miles) away from the prison where Demirtas is being held.

Mahsuni Karaman, a lawyer for Demirtas, said on Twitter that the court had the authority to take a decision in a few days.

"We hope they will do this and sign a ruling that will hold a light to this dark political environment," Karaman added.

Demirtas and HDP MPs including ex-leader Figen Yuksekdag are accused of links to the Kurdistan Workers' Party (PKK), which has waged an insurgency inside Turkey since 1984.

The PKK is proscribed as a terror organisation by Ankara and its Western allies.

The HDP is accused of being a political front for the PKK, but the party denies this and insists it is being targeted because of its strong opposition to Erdogan.

Demirtas was a candidate against Erdogan in the August 2014 presidential elections and led the HDP into parliament for the first time in June 2015.