Turkey's pro-Kurdish Peoples' Democratic Party (HDP) has filed an application at the European Court of Human Rights regarding the continued arrest of its co-leaders , Selahattin Demirtas and Figen Yuksekdag.

The application was filed in Strasbourg on Monday by a delegation including the party's deputy leader Saruhan Oluc, group deputy chairman Filiz Kerestecioglu and MPs Mithat Sancar, Ertugrul Kurkcu and Osman Baydemir.

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In a statement published on Monday, party officials said that the failure of Turkey's Constitutional Court to carry out an investigation into its Yuksekdag and Demirtas' imprisonment has necessitated an application to the European court, the ECHR.

A total of 13 HDP politicians were arrested in November 2016 on terrorism-related charges after their parliamentary immunity was lifted last March . 10 of these MPs, including the party's co-leaders, remain in custody pending trial.

Referendum

In its application to the ECHR, the party argued that the continuing imprisonment of their co-leaders constituted "a violation of the right to freedom and security, freedom of speech and the right to free elections as protected by both the Constitution and the European Convention on Human Rights".

The HDP, the Turkish parliament's third-biggest party, said in its application that since the failed coup attempt in July 2016, 5,471 people have been taken into custody and 1,482 people have been arrested within the scope of operations targeting the HDP and its supporters.

The HDP said that the situation is "particularly critical as Turkey is now heading for a referendum".

President Recep Tayyip Erdogan has approved a constitutional reform bill earlier this month, in a move paving the way for the referendum on an amendments package that seeks to extend his powers.

The referendum is planned to take place on April 16.

READ MORE: Turkey's constitutional reform - All you need to know

The HDP claimed that the continued detention of its co-leaders and MPs aimed to "target and punish the opposition working to organise an effective 'No' campaign during the referendum".

"The process has become increasingly arbitrary and systematic and politically motivated," the party said.

The first hearing of Demirtas will be held on April 28, 12 days after the constitutional referendum.

He faces trial in more than 100 different cases that include charges of "managing a terrorist organisation," and faces over 100 years in jail.

Turkey's government has long been accusing the HDP of being the the outlawed Kurdistan Workers' Party's (PKK) political wing.

The United States and the European Union designate the PKK, an armed group that has been fighting for Kurdish autonomy since 1984, as a "terrorist organisation".

The HDP denies direct links with the PKK and promotes a negotiated end to the Kurdish conflict, which has claimed hundreds of lives since a peace process, once led by Erdogan and the AK party, collapsed in 2015.